The Human Body
(A Curriculum Blueprint)

What follows is an attempt to sketch out an interesting, logical story, by which you and your children can explore the human body and discover how the systems work in a first-handed and active-minded way. This “blueprint” is somewhere between a bullet point outline, and a full set of lesson plans, and it is meant to provide broad guidance while still allowing flexibility to adapt to varying contexts and circumstances. It is my intention to offer a members-only “content library” at some point in the future, which will include some worksheets and slideshows and things, and will eventually include a full set of lesson plans.

During this course on the human body we also learn a lot about animals along the way, by comparing and contrasting them with human beings. If this curriculum were combined with a separate unit on animals, and another on plants, I think the whole thing would make a very good life science course for children in the age range of roughly 8-12. I developed and used it in this way during my years as a science teacher.

Muscles and Bones

In this unit, we systematically map out our machinery of motion, building up our own "map collection," and we discover the power of having a "musculoskeletal system."

Mapping Out the Skeleton

Upper Limbs — Let's start by making a map of our hands and arms. We can discover what many of the bones are like just by feeling under the skin, and we can use x-rays or real skeletons to find the rest. While searching for muscles, sometimes we can find one obvious lumpy muscle, like the one on the front of your upper arm, but most muscles are more mingled and “fabric-like.” We can dissect a chicken wing and study pictures from an atlas of anatomy to help us find all the important muscles. What other kinds of fabric are there besides bones and muscles? We can find many tendons just by feeling under the skin. (We might have confused them with bones earlier.) We can also see in the chicken wing or in the atlas pictures how the tendons grow out of muscles like cords, and how they sew the muscles to the bones at each end, allowing the muscles to pull on the bones and thus rearrange our skeleton. A chicken foot can help us to see exactly what the tendons do. When we dissect a chicken wing, we can see real examples of bone, muscle, and tendon, and we can see some interesting similarities between the chicken wing and a human arm, and also some differences. To record everything we found, we can draw our own map of all the bones and muscles in our “upper limb.” For fun, we could also make toy versions, or “models,” of a finger and an arm.

Lower Limbs — Our legs are different from our arms, but if we compare them to, say, the legs of a spider or the tentacles of an octopus, our legs and arms are quite similar. If we look at the arrangement of the parts and the joints, and in the way that the joints move, we see that our legs and arms have a nearly identical pattern. Are our legs similar to our arms on the inside, too? Let's explore and find out. We can feel for bones near the surface, we can look at skeletons or x-rays to find deeper bones, and we can dissect a chicken leg if we want to compare people to chickens again. And we find an almost identical pattern in the arms and legs: Both limbs start with a “ball and socket” joint, which gives the whole limb a wide range of motion. (Shoulders can move almost any way you want them to, but they can also become “dislocated” more easily. Hips very rarely become dislocated, but they also can't move quite as far as shoulders.) After the ball-and-socket joint that attaches the limb to the body, there is one big stick-bone (femur and humerus), then a folding joint (knee and elbow), a pair of long stick bones (lower leg and forearm), a bunch of little pebble-bones all tied in a bundle (the heel of the foot and the base of the hand), then an array of five stick-bones (the flat of the hand and the instep of the foot), and finally there are arrays of many little stick-bones (fingers and toes). Let's make sure to draw a map of the “lower limb” to record everything we found, and then we can keep it with our map of the arm and hand.

Bones and Muscles in the Head and Torso — What would life be like if we didn't have arms or legs? Our “limbs” are what make us mobile and athletic, and they are made mostly of bones and muscles, with a covering of skin. Our limbs are our main “athletic organs.” Our heads and our bodies (or “trunks” or “torsos”) are for doing other jobs. They are mostly for holding special organs with other purposes. But there are still bones and muscles in the head and torso, too. Unlike the limbs, the bones and muscles here work a lot like protective cases for the special stuff inside, but they can still move a little, especially the neck and waist and face. We can call the bones in our middle parts the “axial” skeleton, as opposed to the “appendicular skeleton” in our limbs, and we can explore the axial skeleton in a similar way. We can feel most of the protective “armor bones” right under the skin, and we can search for the rest with x-rays, skeletons, and pictures from an atlas. (We could also dissect a whole chicken or turkey.) And we can discover some interesting things about our axial skeleton. For one thing, most muscles in the limbs attach to a different bone at each end and their job is to move the bones around, but there are muscles in the face that don't attach to any bones at all. They just grow in skin, and their job is to move the skin around. These muscles help us to talk and make faces. Finally, when we're done with the axial skeleton, we'll make a map of the skull and the axial skeleton to put with our maps of the limbs. Now we're ready to step back and look at the whole thing, at our machinery of motion.

Musculoskeletal System & Vertebrates

Putting It All Together — What would life be like without bones? Our skeletons let us stand up, run around, and push and pull on things. They give us shape and strength and durability, but unlike a shell they still allow us to bend and to move in very many different ways and to be athletic. There are “armor bones” where we need lots of protection (mainly in the head and chest), and there are “stick bones” where we need lots of leverage and power (in the limbs). There are also some “pebble bones” in the wrists and ankles, and “weird-looking bones” in the spine and the face. And the whole skeleton is sewn together very tightly at the joints with strong ligaments, and covered with a “fabric” of muscles and tendons, sort of like powerful clothing, that pulls the skeleton into different shapes and works as our motor. Every time we do something, our muscles move our bones (or face) into a different position. The bones and muscles working together make our “musculoskeletal system” … our machinery for making us strong and athletic. When we tally up the total number of bones we've discovered, we find 200. (Books say 206. There are six more tiny bones hiding somewhere inside another bone. Those have a special job, and they aren't really part of the skeleton. We'll discover those later. Also remember that sometimes individuals have extra bones or missing bones, and you also start out with more bones as a baby, maybe 300, but many of them grow together as you get older. And there are also sesamoid bones. So maybe we should say that most adults have about 200 bones in their skeletons.) How many muscles are there? We can't give an exact number, because muscles are weirder and more variable than bones, but depending on how you count, it's in the ballpark of 600.

Animals With Skeletons — We have already dissected several parts of a chicken, and we found some interesting similarities in the arrangement of the skeleton. What about other animals? We can visit a natural history museum to see more animal skeletons (or if you live in Oklahoma, you can visit the fantastic Museum of Osteology!), and we can look at photos or x-rays of other animal skeletons, and we discover something fascinating: Every animal that has a skeleton is just a variation on a common theme. Horses, cats, dogs, even bats and birds and whales and lizards and frogs — they all have a skeleton inside, with a skull and a spine and usually a ribcage in the middle. Most of them have four limbs, and in every limb there is a similar pattern — they all start with a ball-and-socket joint, followed by a single large stick-bone, followed by smaller stick and pebble bones in a similar pattern. (Aquatic animals are often weirder, but you can still find an underlying similarity if you look. Snakes have no limbs, but the “axial skeleton” is still the same. Each animal modifies the pattern for its own needs, but they all start from the same basic pattern.) Every animal with a skeleton has a “musculoskeletal system” very much like ours. This is one of the reasons why these “skeleton animals” are the most “athletic” of animals, and have conquered the land, the sea, and the sky. They all have this athletic machinery system, this arrangement of bones and muscles, this “musculoskeletal system”, to give them strength and power. The formal name for all of these “skeleton animals” is “vertebrate.” (As we will see later, the only other strategy is to have your bones on the outside and your muscles on the inside. This is how bugs and other arthropods work. Other than vertebrates and arthropods, all animals are just soft, squishy, and fairly un-athletic lumpy things, with only a shell if they have any hard parts at all. Octopuses are fairly intelligent and athletic, and some snails and worms are able to live outside of water, but none of these lumpy animals have really conquered the land, much less the sky. You need a “musculoskeletal system” for that.)

The Jobs of the Head

This unit is mostly about the senses, but with younger students I think it's more interesting just to explore what's inside the head, rather than try to abstract "the senses" and study that as a separate topic. We can discover the idea of "senses" as we go along, and generalizations can come later.

Your head and your torso have bones and muscles like the limbs, but do they work like your limbs? The main job of your head and your torso is not to be strong and to move like your limbs, but mostly to hold and protect other things. Your head and torso mostly hold and protect special organs inside of you that have other special jobs to do. So let's explore the organs inside your head next, and we'll explore the organs inside your torso after that.

Parts of the Head — Let's start by reviewing the names and uses of all the parts on the outside of the head. (Do you know where your philtrum or your glabella are?) After reviewing the outside, we'll start exploring the inside of your head, and we'll try to make a map of everything we find. To explore the inside of a head, some science companies sell a sheep's head, cut in half. We can use that and/or a drawing from an atlas of anatomy to discover what's inside our heads. (And if we use both, we can find some interesting similarities and differences between people and sheep.) We can discover that your brain fills about half of your head (in people at least), inside your “cranium.” The front half of your head, i.e. your face, is full of openings and passageways that eventually go down into your torso, and it also holds some very special organs that let you see and hear the world around you. Let's make sure we have useful names for all the major parts, and let's make a labeled map of the whole thing to put with our map collection.

Jobs of the Head — What do all the parts of the head do? Your face is a little like an entryway where you put things in, like air and food and water. But those things all end up inside your torso, so let's talk about those things later, when we start exploring the torso organs. Another very important job of our heads is to hold our eyes and ears, so we can learn about the world around us. [Our heads also hold our brains, and the function of the brain may be “common knowledge” nowadays, but it isn't immediately obvious. Some ancient scientists used to think that the top of the head was like a radiator for cooling the “internal fire,” so that we can stay warm inside without burning up in our own fire. They thought that the soul might reside somewhere in the chest. I don't think you should contradict children if they say that the brain is for thinking and knowing, but it's worth asking “How can you be sure of that?” and letting them know that other people used to believe differently. Whenever you can try to break children of the habit of automatically accepting what they are told, and instead teach them how to “chew” on things and how to ask useful questions — especially in science class — I approve.] What are the different ways we can learn about the world? We see things (using our eyes), we hear things (using our ears), we can smell things (using our nose), we can tell that different things taste differently (using our tongue), and we can tell whether things are hot or cold, smooth or rough, by touching them (with our skin). We have several different “senses” for telling us about the world in different ways, and for each one we have a special “sense organ” that provides the information. And most of these special organs are in our heads. (We also become aware of things like hunger, when we lose our balance, and when our muscles are tired or sore. We have some “senses” that tell us about the position and condition inside our own bodies. Should we count these as “internal senses” and the others as “external senses”?) The eyes and the ears are probably the most sophisticated and wonderful sense organs, because they tell us so much about the world. You might also notice that they are the only ways we can learn about the world far away from our bodies. All of the other “external senses” need to have something from the world touch our skin somewhere in order for us to learn about it, but our eyes and ears can tell us about things far away, and they are much more complicated than just sensitive skin. [In a way, even our eyes and ears require something to “touch” our bodies, but seeing that this is true is beyond our ability to discuss here. We can discover that in future years when we study sound and light.]

Since the eyes and ears are so complex and so important, let's study the eyes and ears specially, and then we'll study the rest of the head after that.

The Eye and Vision

The Parts of the Eye — We can see some very pretty parts just by looking closely at the outside of someone else's eyes. By trying to look through the “window” or “pupil”, we can also guess at a few things that we might find inside. We can shine a light into someone's eye through the “window” or pupil, and see red at the back, so maybe there's pink skin at the back? We can turn and roll the eye in different directions, so it is probably a “ball”, and there are probably muscles around it to make it move, and maybe also some fabric to help hold it in place? Could we find all of these things if we could dissect an eyeball?

How to Cast a Picture — Have you ever noticed this fun trick you can do with a magnifying glass? [I found it useful to introduce this as if it were just a random off-topic fun project, before doing the eyeball dissection. That way, many of the students can experience the joy of recognition when they see on their own the similarity between eyes and lens-projections. But you could also demonstrate this principle of operation after the dissection.] If you hold up a magnifying glass and point it at a window and put a card or paper on the other side, and then play around with how far apart you hold them, you can magically make a picture of the window appear on the card! [A distant outdoor window in an otherwise dark room is best. You can also make images of lamps and light bulbs if you don't have a bright window. For more details, you might find this project to be helpful.] Somehow the lens can “throw” a picture of the window over there onto the card over here. By the way, did you notice a similarity to the eyeball? There's a dark “room,” with a pretty, curved clear thing at one side, and maybe a “screen” of some kind at the other? Do you suppose the eyeball works in a similar way?

Eyeball Dissection — You can order cow eyes cheaply from science supply companies, and they are pretty easy to dissect, so let's cut one open and find out what's inside! We can discover the “pink skin” (retina) at the back, the “jelly” inside (vitreous humor), and an actual “window pane” in the pupil opening which looks just like a second “magnifying lens.” (We can call this the “pupil” if we want to, but the official name is “crystalline lens.”) When we are done, we can look at the picture of a human eyeball in the atlas of anatomy to see if there are any differences between cows and people, and then we can make a nice, neat map of all the parts of an eyeball.

How Does the Eye Work? — Now that we have discovered the map of the eye, we can try to figure out how it works. It seems to work like the magnifying-glass-and-screen. The cornea (helped by the “pupil” or “crystalline lens”) casts a picture onto the “screen” or retina, and then the pink skin of the retina “feels” the image somehow? Other than muscles, the only thing that connects the eye to anything else is the giant “optic nerve”, which grows right out of the retina and goes to the brain behind it, so maybe after “feeling” the image, the pink skin “tells” the brain somehow through the optic nerve? [Depending on time and interest, this is a good time to expand on the subject of “vision”, by having some fun with things like focusing, depth perception, blind spots, afterimages, and perceptual illusions, and discussing what these things can tell us about how the eye works. You could also make a model, or a "toy eye."]

Animal Eyes — Most animals that are built like us (i.e. vertebrates) have eyeballs that are like ours. But there are some differences. What about animals that can see at night? Have you ever noticed that pupils come in different shapes? Have you ever noticed that some animals have both eyes facing forward, like us, but other animals have their eyes on the sides of their heads? [Depending on the pictures or live animals that you have available to study and how much time you want to devote to this, you can discover things like: Birds and mammals have “fancy” eyes like us, but “simpler” vertebrates like frogs and fish usually have simpler eyes, and “weirder” animals like flies and praying mantises have weirder eyes. You can also notice that there's a trade-off between depth perception and peripheral vision, and different animals prefer one or the other: “Eyes in front, likes to hunt, eyes on the side, likes to hide.” Owls and other nocturnal animals have large eyes, and sometimes other special features, that help them see better in the dark.]

The Ear and Hearing

The Outer Ear and the Eardrum — As we usually do, let's start by reviewing and summarizing what we can learn from the outside. Which do you think is more important for hearing, the “ear” on the outside, or the hole? Sometimes people are missing their outer ears, and they can still hear just fine. The “external ear” helps to make sounds a little louder, but not much. The important equipment must be inside the hole. What's inside the hole? Sometimes we can look inside someone's ear and see a wall (“eardrum”) at the end of the tunnel (“ear canal”). (Looking inside an ear isn't easy, which is why doctors have a special tool. But you can buy one of these tools yourself, and they aren't too expensive. You can also find plenty of photographs online, or look up the corresponding pictures in your atlas of anatomy.) Is that all there is? Is there anything else on the other side of the wall? We can see that the eardrum is a little translucent, and it looks as if there might be another chamber or cave on the other side of the wall. There are also ways we can tell that the eardrum moves when there are sounds. (For one thing, we can make toys that look like eardrums, and we can notice that sometimes they vibrate when there are sounds nearby.) To learn more, we need to explore deeper inside, on the other side of the eardrum.

Exploring Behind the Eardrum — Whatever is on the other side must occupy a “cave” or hollow inside the skull bones. This is not something we can explore easily in a classroom. Sometimes you can find pictures of a skull that someone has carefully cut into pieces, but mostly we'll just have to make do with studying a picture from an atlas of anatomy. First of all, we discover that there are actually two caves. (We officially call these the “middle ear” and the “inner ear.”) We also discover that the first cave has tiny bones inside! These “ossicles” are linked together, like a “mini skeleton,” touching the eardrum on one end, and going to the second cave on the other end. When the eardrum wiggles, they must make the inner cave wiggle, too. The inner cave is really complicated, but somebody made a metal cast by pouring liquid metal into this “cave” in an old skull, and he made a very good “map” of the entire inner cave. We can see that there are three loops (“semicircular canals”) attached to a coil (“cochlea”). According to the atlas picture, there is also a big nerve, similar to the optic nerve, going from the inner ear to the brain.

How the Ear Works [This is harder to “chew on” in a thoroughly evidence-based way, especially with young kids, but you can make conjectures and draw analogies. Think of spinning a jar or a bucket with water inside. If you suddenly start spinning the bucket, the water stays in place at first, and takes a while to get up to speed with the bucket. If you suddenly stop spinning, the water keeps going for a little while, and takes a while to stop. The semicircular canals are like that, and any sloshing around of water in these circles gives us information about the motion of our head. Why do we have three of these loops? Notice that they are more-or-less perpendicular to each other, so they can tell us about tilting or turning in 3D, i.e. in any direction. Microscope pictures can show us that the cochlea has a tiny patch of “hairy skin” inside, and if the water inside moves, maybe the hairs move like seaweed swaying in the currents, and maybe we can “feel” this motion in a similar way to when we feel tickling of the hairs on our head or arms? What about the ossicles? What is the middle ear for? Why not just have sound go straight from the eardrum to the cochlea? If your students know about “leverage,” you can suppose that maybe these bones act like magnifiers, and make the tiny motions of the eardrum into bigger motions in the cochlea, so that we can hear better. As usual, we'll summarize everything we've learned by making a nice final map of the ear and the “ear caves” to put with our map collection. Time permitting, this is also a good time to have some fun with things like the benefit of having two ears (to perceive the direction of sounds), with Eustachian tubes and “ear popping”, and so on.]

Hearing, Voices, and Animals — What's the difference between speaking, singing, and whispering? What's the difference between vowels and consonants? Have you ever noticed that there are different kinds of consonants (fricatives, plosives, nasals)? Have you ever noticed that the animals with the best voices are often the animals with the best hearing ... and these animals are also the best athletes (i.e. mammals and birds)? These animals are the “best breathers.” [As we will discover later, mammals and birds have special “breathing muscles” (diaphragms), and the largest, heaviest lungs. They have the best “breathing equipment,” which supports their superior “athletic equipment,” including the “internal fire” that keeps them warm. And this superior breathing equipment also gives them much better voices.] What about animals with different kinds of ears? How do different kinds of ears help animals in different ways?

The Entryways. Tasting & Smelling

Apart from seeing and hearing (and speaking), the other job of your face is to be a doorway. Your face holds the openings where food and water and air go in. And both of your openings have “quality control senses” to check what's coming in. Your sense of taste tells you about the food you put in your mouth, and your sense of smell tells you about the air coming in your nose. Since we just finished talking about seeing and hearing, this might be a good time to make a few comments about tasting and smelling as well. Let's save teeth and swallowing until we get to food and digestion and the organs in your torso, and right now let's talk a little about how your nose and your tongue work. (If you don't remember from our exploration of the head, notice that there are two main chambers: the “mouth” or “oral cavity”, and the “nose” or “nasal cavity” along with the attached “paranasal sinuses.” We'll start with smelling and the nose, and then we'll talk about tasting and the tongue.)

The Nose and the Sinuses — What are all these weird complicated chambers for? And why is there hair inside the nose? Engines and central-air furnaces and all things that need to “breathe” usually have “air cleaners” to help keep the incoming air clean. Maybe the nose is like an air filter, keeping the air from going straight to the lungs, and helping to make sure the air is clean before it gets to the lungs? What might happen if we breathed really dry air, or really cold air, or really hot air through our mouth for a long time? Maybe the lungs deep inside our ribcage are sensitive things, and they need to be protected from harsh air, and this is what the nose is for? Smelling apparently works much like touch — fumes and vapors and odors just need to touch the inside of the nose? If we carefully inspect a brain, we may discover the places where the optic nerve and auditory nerves were attached. We can also discover a third very large nerve (the olfactory nerve) leading from the brain towards the top of the nose, and if we examine a skull (or a picture of one in an atlas of anatomy), we discover tiny perforations (the cribriform plate), where this nerve passes through the bony wall of the cranium into the skin of the nose. Apparently, odors touch the skin in your nose, the skin “feels” the odors somehow, and then the skin tells the brain through the olfactory nerve? Can we name different scents, like we name colors or sounds? Not really, but we can notice there are some similarities between smells and tastes, and that “good” or pleasing aromas often come from healthy things, while “bad” or displeasing odors often come from unhealthy things.

The Tongue — Sometimes you can buy “beef tongue” at your local deli or meat counter. If so, you might discover that it is made of mostly muscle, with a covering of skin. But the skin is a little different from the skin on the rest of your body. There is no hair, and instead there are these weird “buds.” And we become aware of tastes when foods touch these buds. [There is a good opportunity here for a lesson on not taking for granted everything you read, even in science. Many older science books, and some not so old, show a “map” of different regions on the tongue, and claim that one area is for salty tastes, another for sweet, and so on. And they instruct you to do an experiment to discover this. But apparently the writers of these books never actually tried the experiment themselves, because it doesn't work. What seems to have happened is that a German scientist published a paper long ago claiming that sometimes different areas of the tongue can sense some flavors a LITTLE more strongly than other flavors. He did not claim that there were exclusive zones on the tongue, each responsible for its own flavor. And to the best of my knowledge, no doctor or professional scientist takes this “taste zone map” seriously. But somehow this map got into children's science textbooks, and then as textbook authors often do they all copied each other, and the “tongue taste map” became a common science “fact.” With my students, we did the experiment carefully, we discovered no differences between different parts of the tongue, and then I told them this story as a lesson on judging and thinking for yourself.]

The Brain, Nerves, and Behavior

You remember faces you've seen before, and you remember tunes you've heard before. Is there a “memory organ” where these memories are stored? If so, this organ would have to be connected to your eyes and ears, so that you can remember what you've seen and heard. And your muscles move you around in response to things you see and hear. Sometimes your body responds for you, as with instant reflexes, and other times you yourself choose what to do based on what you see and hear around you. But in either case, somehow your body tells your muscles what to do in response to things your sense organs report. Is there a “deciding organ” somewhere? You also struggle to learn complex skills like riding a bicycle or playing a piano, but once you've learned the skill it becomes easy. You remember how to do it. Is there a “muscle memory organ” somewhere that remembers how to move your muscles in complex coordinated ways? It seems like there must be a “master control organ” somewhere for remembering things you've seen before, for remembering skills you've learned, and for deciding what to do based on what you see and hear. But which organ could this be? If there is one, it would have to be connected to all of your sense organs and to all of your muscles (or at least the voluntary muscles, if not all of them). The heart is connected to everything in the body by tubes; Could the “master organ” be the heart? We've also already discovered that the eyes, the ears, and the nose are connected by big cables or “nerves” to the brain. Is the brain connected to other body parts as well, maybe by wires too small to see? Could the “master organ” be the brain?

Discovering the “Nerve Tree” — Sometimes if you hunt very carefully in large joints (for example, the hip joint of a chicken), you might find really tough “wires” running through the joint. These seem like tendons, because they are long and thin and very strong, but they don't grow out of muscles (or at least not that we can see), and they don't connect to bone. We call these wires “nerves,” from a Greek word meaning “tendon” or “sinew.” Tracing them is extremely difficult. We might find a few large examples in dissections if we look hard, but the vast majority are tiny, even microscopic, and hard to find. We'll have to rely on our atlas of anatomy to trace these nerves. (You could also visit a plastination museum, if there is one nearby. They usually have a complete “nervous system” on display that you can study.) You've probably already noticed the “yellow wires” in so many of the atlas pictures we've seen before. These are “nerves,” and they are arranged in a sort of “tree”, with the spinal cord like the trunk of the tree. All of the big nerves come from the spinal cord and split into smaller and smaller nerves, like the branches of a tree, and they run everywhere throughout the body. So not only is the brain connected to the eyes and ears, it is connected to everything, including all of the sense organs and all of the muscles, both voluntary and involuntary. The brain is at the core of a “nerve tree” running through the entire body. It connects (through microscopic nerves) to every muscle, and to every sense organ. (And as we've seen, the nerves that connect the brain to the eyes and ears and nose are huge and easy to see.)

The Job of the Brain — Some ancient scientists thought that the brain was for keeping us from burning up: We require fuel, just like a flame, we can be suffocated, just like a flame, and we have our own warmth inside, just like a flame. And the things that qualify as “food” can almost always burn (at least after you dry them out). So maybe our warmth comes from burning our food. But we are much cooler than a flame. And the brain seems to have an unusually large share of the “blood tubes” running to it. So maybe the brain is like a radiator for cooling off our blood and keeping our “flame” from becoming too hot. (And when something goes wrong, maybe that explains why we get “red-faced”, or have a fever, and our head feels hot.) But by carefully discovering and following the nerves, we now realize that the brain is not a “blood cooler,” it is instead the “master organ” for coordinating everything we do. It sits at the center of the “nerve tree” which connects it to every sense organ and to every muscle. It is for receiving information from the senses, deciding what to do, and sending commands out to the muscles. (And this also helps explain why, unlike any other organ, it is almost completely encased in “armor bones.” It is extremely important, but it doesn't need to move in any way, so it has its own rigid “safe” or “vault” to live in.)

Optional Discussion Points [What's the difference between the cerebrum and cerebellum? I found a story in an old psychology book by William James describing what happens when you remove the entire brain from a frog, or you remove only the cerebrum and leave the cerebellum. I told this story, somewhat modified, to my kids. In a nutshell: With no brain, the frog still has automatic reflexes, but no coordinated action at all. The spinal cord can take care of automatic reflexes by itself, but the brain is necessary for all other action. With a cerebellum but no cerebrum, the frog can swim and climb, but only in response to immediate stimuli, like being immersed or sitting on a slope. It can't recognize food or danger and it won't swim or climb towards or away from anything. The cerebellum apparently “remembers” how to coordinate muscles into complicated skills like swimming or climbing or playing the piano, while the cerebrum handles all “higher functions.” There is also the story of Phineas Gage, which shows that you can still live and be functional with a damaged cerebrum, although it may cause you to behave in strange ways. Is there really a difference between the “right brain” and the “left brain”? Do different parts of the brain do different things? This is something I normally discussed only with junior high students, but if time and interest permit, it could be fun to explore Broca's area, Wernicke's area, the motor cortex, the sensory cortex, etc. Like the “tongue taste map,” the popular notion of “right brain vs left brain” is greatly distorted and oversimplified. It may be interesting and useful to notice that different people have different character traits, but these character traits have little or nothing to do with which hemisphere of their cerebrum they are using.]

The Nervous System — So those are the main jobs of your head. It holds your openings for everything you put into your body, along with a couple of sense organs that work like “quality checkers” for everything you put in. It also holds your precious “distance senses” which tell you about the world around you. And it holds your “master organ.” It holds your brain, which is connected to every other part of your body, which remembers things you've seen and heard and skills that you've learned, and which decides what to do and what orders to send to the muscles based on your senses and your memories. If your limbs are the most valuable components of your machinery or your “musculoskeletal system,” then your head is your “awareness and control” center. Your head contains the most important components of your “nervous system.” And every animal, even the simplest, has a nervous system. It has to. (Some of the very simplest animals have tiny brains, or no brains at all, but they all have nerves, and at least some kind of rudimentary sensory capability. If it has muscles, it has to have a nervous system to tell the muscles what to do.) Every animal that can move around and do things needs to know which way to go and what to do. Every animal that can move has a nervous system. [One interesting exception would be the sponge. Sponges are technically classified as animals, but their anatomy and lifestyle are more like those of plants.]

Internal Organs

In this unit, we explore the organs in the torso, we try to figure out what happens to all of the "material inputs" and where the "waste outputs" come from, and we map out the entire "nutrition factory" housed inside the torso.

Think of your whole body in three pieces: your “body” (or more precisely your “torso”) in the middle, with your head stuck on top, and your set of limbs sticking out from the four corners. Do you see how each part is built specially to do a special job? Your limbs are all bone and muscle, and they are your most important “motion tools” or “athletic organs.” Your head has most of your sense organs, especially the eyes and ears, and it also holds your “master organ,” your brain. Your head is your “information and control center.” Your torso is where all of your material inputs (food, water, and air) end up, it holds all of your “internal organs” for working with these materials, and it is where your wastes come from. Your torso is like your “material processing factory.” Food and water and air go in, something happens to them that keeps you healthy and active and energized, and then wastes come out.

We already studied your “athletic system” (or officially your “musculoskeletal system”). We also studied your “sensation and control” system (or officially your “nervous system”). And we have a map collection covering both of these systems. Now let's study the internal organs in your torso, and let's see if we can make a map of the organs in your “nutrition system,” and try to figure out what's going on in there.

Exploring the Organ Bundle

Before we begin, let's notice an important difference between the upper and lower halves of your torso. In people (and in all animals that resemble people), the upper half is surrounded by a bony cage, and it holds your more precious organs for air and blood. The lower half is only surrounded by some sheets of muscle, and it holds the organs for food and water. Officially, we call the top the “chest” or “thorax,” and the bottom is the “abdomen.”

A Sketch of the Big Picture [Normally in science I like starting with direct and personal observations, and saving the “cartoons” for later as summaries or concretizations or “maps” of things we have already discovered. However, this is one case where I think it is better to start with an orienting “preview” first, and after that we dissect an animal's torso to do our own personal discovery activity. The contents of a torso are so complex and specialized that starting with a dissection would be too random and messy and the children would miss all kinds of interesting details in the mess. Also, the major organs in the torso are more or less “common knowledge,” and the kids will probably already know the names of a few organs anyway. So I think it is better for kids to have a basic familiarity with the layout before we begin the dissection, and this will make the actual dissection more rewarding. They can recognize and confirm the presence of organs they already “know,” they can personally experience the texture and appearance of these organs, and they can discover subtle details they would have missed in the mess otherwise — like the connections between organs, or the differences between pig and human anatomy. Also, I place the highest importance on showing kids how to reason from direct evidence to hidden conclusions, but sometimes it can be useful to give kids experience with reasoning in the other direction. Instead of saying “let's look and figure out,” it can be important to sometimes say: “Here's what other people say. Let's see if it makes sense. Let's see if we should agree or not.” So with that in mind...] Let's begin by looking at some cartoons of what “everybody says” is in your torso. We'll remember the biggest organs and their positions, and then we'll get a closer look at real organs when we dissect an animal. This way we can look for differences between real anatomy and cartoons, and we can look for similarities and differences between pigs and people. [This is also a good time to have some fun talking about ways in which everybody is the same, and ways in which people can be different. Sometimes people have backwards organs (situs inversus), stomachs come in a fairly wide range of shapes and distortions, and there are numerous other examples of human variation.]

Fetal Pig Dissection — Like eyeballs, fetal pigs are readily available as a byproduct of the meat industry. These would have been wasted otherwise, but sometimes the companies save them and sell them to science teachers. They can give us a good opportunity to see and learn. We can look inside a real torso, and see how real organs are arranged and connected and what they look like. [Incidentally, I was worried about squeamishness when I first started doing these dissections, but now I think squeamishness is a learned trait. I found that young children loved the dissections almost to a child, and the only problems I had with students and dissections were with teenage girls. For most children, the fetal pig dissection was one of the highlights of the year. Also, the quality of the specimen can make a huge difference in how “disgusting” it is, so I definitely recommend purchasing from a reputable company, like Carolina Biological Supply.] As we do the dissection, we can discover that the pig's organs are pretty much like we expected, but we also see many interesting details. The diaphragm “seals off” the bottom of the ribcage, helping to make the thorax and abdomen into completely separate chambers. We see that the lungs and heart are tied together by a “tube tangle” into a single “bundle” of organs in the thorax, and we see that the lungs are not like bags or sacs. They are heavy, dense, and spongy. [If you are careful, you may be able to demonstrate that the lungs are inflatable by cutting one off at the hilum and attempting to squirt a little air into the bronchus with a small syringe.] We can also discover a big muscular lump at the bottom of the stomach (the pyloric valve), we see that the intestines in the fetal pig are a bit different from those in a person, we find “coral-like” folds of intestine that hold the loops in place, and we find “rays” of threads or tubes or something inside these folds of fabric. We also notice a couple of major tubes running up and down along the spine (the aorta and vena cava), like the major “freeways” of the tube system. We could also notice that the umbilical cord split into two pieces inside the pig's body, one of them going to the liver and the other to the bladder.

Digesting Food

Now that we have a pretty good map of all the special organs inside the torso, let's try to work our way through the system, and figure out how it all works. We'll start with the “food tube” and see what happens to food and water after you swallow. Let's follow food as it goes through the system, and we'll make a separate map of this “digestive system.” Maybe we can find liver or tripe or maybe even intestines in the grocery store, so we can take another good look at those specific organs, and we can examine the pictures in an atlas of anatomy for a closer look at some of the specialty organs, and to compare animals to people.

The Mouth — How many teeth do you have? Should we count them as bones? Have you ever noticed that teeth come in different kinds? (This is one thing that makes people and other mammals different from “lower organisms,” and even from “lower” vertebrates. We have specialized teeth, with different shapes for different jobs. The teeth in the jaws of lizards and amphibians are all the same, like the teeth of a saw blade.) Why do we chew? Why do we salivate? How do we get food to go down the esophagus without having any go into the lungs? [I found some fascinating fluoroscope videos on youTube of people chewing and swallowing, which helped to illustrate the complex actions of swallowing, and how the epiglottis closes off the trachea when you swallow.]

The Food Tube — Notice that the esophagus, the stomach, the small intestine, and the large intestine are all connected end-to-end, forming one single long tube. You have a single “food tube” running all the way through your body, from your mouth at one end, to the place where wastes come out at the other end. The official name for this “food tube” is the “alimentary canal.” (And the official name for the “exit” is the “anus.”) How do horses drink from puddles on the ground? Have you ever tried to drink through a straw while doing a handstand? Life would be difficult if we had to rely on gravity to make things go into our stomachs, but our bodies have a way to force food and water from the mouth into the stomach, even if you are upside-down. How? Notice that the walls of the entire food tube are made of layers of muscle. What would happen if these muscles all squeezed at once? If all the muscles of the stomach squeezed at the same time, it would squirt out whatever was inside. That's probably how you throw up. If you study the stomach muscles in an atlas of anatomy, you'll notice that there are different layers, with fibers going in different directions. Maybe using these muscles one at a time is what churns your food around and makes your stomach make noises? Now, what would happen if all of the muscles of the esophagus didn't squeeze at the same time, but in order, in a synchronized “wave”? Wouldn't that force food to go down into the stomach? The esophagus can force food and water to go into the stomach, even if you are upside-down, by squeezing in waves (“peristalsis”). The entire food tube has walls of muscle, and these muscles are what move food around in the tubes. Now, what happens to food once it gets to the stomach? One clue is what comes out when you throw up. One scientist (Lazzaro Spallanzani) fed meat to birds in perforated containers, then pulled it back up again by strings. Once there was someone who was shot in the stomach and the hole never healed completely, so people could look in and watch what happens. (That's the story of St. Martin's Fistula.) In any case, it is clear that there are some pretty powerful digestive juices down there, and they cause the food to disintegrate, presumably so that your body can absorb the nutrients better. In the stomach, the food is turned into a creamy white substance (chyme), and then when it's ready to go further, the tight knot of muscle at the end of the stomach (“pyloric valve” — you may remember this from the pig dissection) opens and lets the food go on into the intestines. The intestines are apparently for “soaking out” the nutrients from the chyme, but where do the nutrients go from there? We can see in the pig dissection and in the atlas pictures that the intestines are connected by a rich network of blood vessels that all run to the liver. (That's the “river to the liver,” or the hepatic portal vein system.) Apparently, the intestines “soak out” the nutrients from the chyme and send them to the liver. Then the un-nutritious wastes and leftovers go out the end of the large intestine.

Helper Organs — What does the liver do? The liver is connected to the major tubes running along the spine (the aorta and the vena cava), especially the vena cava, so it might be like a “nutrient sponge.” Nutrition comes in to your body in big bursts, but your body probably needs them more or less constantly throughout the day. So maybe the liver stores the food nutrients from the intestines, and then it lets them go out gradually and steadily through the blood vessels to the rest of the body? What about the pancreas and the gall bladder? We noticed in the dissection that the first section of the intestines was extra tough and thick (the “duodenum”), maybe to protect it from stomach acids? Then why aren't the rest of the intestines tough? Maybe that little portal in the duodenum is for injecting “anti-acids,” to undo what the stomach did, and to help protect the intestines from being digested? The kidneys have no direct connection to the rest of the organs in the abdomen, so they don't seem like they have much to do with digestion, at least not directly. We'll take a special look at the kidneys later.

The Blood and Air Organs

The food organs and the “digestive system” lie mostly in the abdomen. The job of the abdominal organs is digestion. What about the thorax? Let's see if we can get a better look at the heart and lungs and figure out what they do, and then we'll see if we can put all of the puzzle pieces together, and make a complete map of the entire “nutrition factory” in the torso.

The Thoracic Bundle — The blood-and-air organs inside the ribcage come in a tightly tied-up and sewn-together bundle, with tubes tying the organs together, and the trachea sticking out the top. We can get a closer look at this bundle by studying the pictures in an atlas of anatomy, and we can also order sheep pluck from a science company. [A “sheep pluck” can be fascinating to study and dissect, but it depends somewhat on the quality of the specimen. Sometimes I would try blowing into the open end of the trachea to inflate the lungs. Sometimes it worked. You can also see how the lungs have lobes and that the lungs are connected to the heart through a tough “doorway” or “hilum” containing all the big tubes. If you cut through the hilum you can detach a lung and try looking into the tubes.]

Lungs and Breathing — One thing we should have noticed, either in a fetal pig dissection or a sheep pluck dissection, is that the lungs are not like hollow sacks or bags. They are very heavy sponges for “soaking up” the air somehow. There are also three kinds of tubes running through the lungs. Some people have figured out how to pour plastic into the three kinds of tubes in a lung, and then they dissolved away the lungs, leaving the plastic to show us what the tunnel system looks like. They made a resin cast of the passages in the lungs. We can see from such a cast (either in person in a plastination museum, or in photographs) that the three kinds of tubes all make a branching “tree-like” system. All three large tubes that come in through the hilum spread into the lobes, branching into more and more smaller and smaller tubes, and all three “tube trees” mingle their branches in the spongy material of the lungs. The lungs are basically made of three overlapping “tube trees,” and they are apparently for “soaking together” blood and air somehow? The cartilage tubes connect to the trachea and are obviously for carrying air. The two fabric tubes connect to the heart. If we assume that these two kinds of tubes carry blood to and from the heart, then maybe the lungs take blood from the heart, mix in fresh air, and give the blood back to the heart? An easier question to answer is how breathing works mechanically. We've discovered the dome-shaped muscle (diaphragm) underneath the lungs, and you can notice that your belly swells when you inhale. Apparently your diaphragm pulls down on the lungs from below when you want to inhale, and this sucks air in to your lungs through the windpipe (and also pushes down on your guts at the same time). We've also found some miscellaneous smaller muscles around the ribcage, and the layers of muscles around your abdomen that can squeeze your abdomen when you want to squeeze something out of you. These muscles help when you want to take an extra deep breath, or to exhale forcefully, to blow out a candle for example. If we want, we can model how breathing works by making a toy lung.

[The next two sections mainly concern the discovery of the circulatory system. With middle school students, I would usually go through this discovery historically and in detail, from the ideas of the Ancient Greeks, to those of Galen, to the proofs and the reasoning of William Harvey. It was a fantastic “thinking exercise.” But to be frank, I never did find a way of approaching this subject with elementary students with which I was totally satisfied. I usually spent more time on “fun” stuff like finding different pulse points and seeing what happens to your pulse under different circumstances. Regarding theory, I just sketched out some of the main pieces of evidence, and some of the main alternatives and questions, as a preview or a foreshadowing of what they would cover in middle school. What follows is a skimming-over of a few of the main ideas.]

Veins, Arteries, and Your Pulse — You can feel your pulse in numerous places on your body … but why are these places almost always at your joints? If you've been studying pictures from an atlas of anatomy for a while, you've probably noticed that we normally find two sets of tubes running through the body. In the pictures, one is normally colored red and the other blue. In several dissections, you may have noticed two sets of tubes, more or less parallel to each other. One kind of tube usually has thicker, almost muscular walls, while the other is fairly limp, with thin walls. And both of these “tube trees” have giant “trunks” that run up and down along the spine. The muscular set we call “arteries,” and the “trunk” of the artery tree in your torso is the “aorta.” The limp set of tubes we call “veins” and the “trunk” is the vena cava. What runs through these two tube-tree systems? When ancient scientists would carefully dissect cadavers, they would usually find that the muscular set of tubes was empty, and the limp set was puffed full of blood, so maybe veins are for distributing blood from the liver to the rest of the body, and arteries are for distributing air from the lungs to the rest of the body? Other than muscular walls, there is another significant difference between veins and arteries: We can often see veins at the surface, but arteries are always hidden deep under layers of flesh. They only come near the surface in joints, where there is no muscle to protect them. So it seems like when we feel our pulse, we are feeling a throbbing in the entire “artery tube system.” (It would be interesting to see if all pulse points pulse in unison, or if there is a slight delay in the extremities.) The veins seem like they might be rooted in the liver, and they might be for carrying blood from where it is made in the liver to the rest of the body? And the arteries might be for carrying air from the lungs to the rest of the body, and you feel your pulse when there is a puff of air in the arteries?

The Heart and the Recycling of Blood — Sometimes you can buy whole beef or sheep or pig hearts at a meat counter. If you can find one, it makes a great dissection project. You can also study the pictures of a heart in an atlas of anatomy. At a rough glance, the heart is a hollow muscular organ, and the caves inside are connected to the tubes that come out the top. What's going to happen if the muscles of the heart squeeze? Ancient scientists thought that maybe the heart works like the lungs — when it squeezes it forces air out into the muscular arteries, and when it relaxes the arteries squeeze and force the air back in to the heart. The “puffing” or “breathing” of air into your arteries is what you feel as your pulse. Your heart sends air from your chest to the rest of your body in an ebb-and-flow, the same way that air enters and leaves your chest from the outside world. But the tube system seems awfully complex and tangled if that is all there is to it. And if you take a closer look inside a heart, you discover something important. The heart has curious loose bits of fabric inside. If we think about how these pieces of fabric are going to move, we see that they are flaps, or one-way valves. Whatever substance is inside the heart, it can't ebb-and-flow, because the flaps only allow it to go one way. Furthermore, if we carefully trace the tubes in the “tangle” by studying the pictures in an atlas, we see that the lungs are merely a detour for the blood, in which the blood is “air-ified” somehow. The untangled “tube tangle” looks like this: Body → Vena Cava → Right Ventricle → Lungs → Left Ventricle → Aorta → Body. And these things finally help us to see that the entire heart/lung package is a one-way street for blood. Blood always comes IN to the heart from the body through the vena cava, it is refreshed somehow in the lungs, and then it goes back OUT to the body through the aorta. So the thorax in mammals is an always-running blood pump and blood conditioner for energizing the rest of the body. But if the blood is constantly being recycled or recirculated throughout the body, how does the blood in the body cross over from the ends of the arteries into the ends of the veins so it can come back to the chest again? Well, why can't it just seep through the fabric of the body as if through a sponge? Why can't it seep through tiny pores or invisible tubes in the same way it seeps through the spongy material of the lungs (or the kidneys)? [And if you have a microscope, you may be able to discover exactly such tiny connecting vessels: capillary blood vessels that serve as a bridge between the ends of the arteries and the ends of the veins.]

The Nutrition System

The Circulatory System — So the always-running “engine” in your chest is for taking blood from the body, sending it on a refreshing detour through the lungs, and then pumping it back to the body in a continuous never-ending loop. Your entire “nutrition factory” in your torso has a conveyor belt running through your internal organs and through your entire body, and this “conveyor belt” is always flowing. But why would the blood need to be recycled or recirculated? Why not just have the digestive system make blood from your food, maybe mix it with a little air from the lungs, and then send it out to the muscles and to the rest of your body as it is needed? Why does your body put so much work into sending the same material around and around, over and over again? Any ideas? Also, why do the arteries have muscular walls? If the artery muscles are not for squeezing air back to the heart, then what are they for?

Why Do We Breathe? — One puzzle we haven't figured out yet is why we breathe. What's the point of taking in air, only to breathe it right back out again? How exactly does air “refresh” the blood? [We aren't ready for the detailed chemistry of oxygen and carbon dioxide yet, at least not in an age-appropriate, evidence-based way, but we can notice some important similarities and differences with flames, and we can make some conjectures. Both flames and people need fuel, and both can be suffocated. If you enclose a mouse and a candle flame in the same sealed container, they will both start to fade and show distress at the same time, and if you take off the lid and let in fresh air before they go out, they will both revive together. Warm-blooded animals and flames have a lot in common, including the need to breathe. There seems to be an important difference between “stale air” and “fresh air”, and it's a life-and-death difference to both flames and animals. Maybe we make our own warmth and energy by “burning” our fuel, in the same way a flame does, and maybe we need to breathe for the same reason a flame needs fresh air? Maybe we need to breathe to carry away the “stale air” (or a harmful portion of stale air) and to bring in a constant supply of “fresh air” (or a vital ingredient in fresh air), and to thus keep our internal flame going? Maybe we need to breathe in order to ventilate our internal fire?]

Kidneys — There is one big organ we haven't talked about yet. What are the kidneys for? We can study the pictures in an atlas of anatomy, and we can also buy beef kidneys sometimes at meat counters, and then we can dissect a real kidney. We discover that kidneys are built a lot like lungs. They are both heavy spongy organs with three overlapping “tube trees” that pass in and out of the organ through a “hilum.” In the kidneys, two of these “tube trees” connect to the aorta and vena cava, and one “tree” of tubes goes to the bladder. And the branches of all three “tube trees” mingle in the spongy material of the kidneys. The lungs apparently take stale air (or “carbon dioxide” if you want to be technical) out of the blood coming in through one set of blood tubes, and they put fresh air (or “oxygen”) into the blood going out through the other tubes, while the air comes and goes through the trachea. The kidneys apparently work in a similar way: They take wastes (urine) out of the blood coming in (in the arteries), they put clean blood back into the veins going out, and then they send the wastes that they removed from the blood to the bladder through the ureters. They are like blood-cleaners or blood-filters. Maybe this is one benefit of having circulation, so that your circulation can work like a “garbage collection system” that is always cleaning your body on the inside? [Kidneys also help to balance or regulate certain nutrient levels in the blood, and the liver also does a lot of “chemical processing,” turning harmful chemicals into harmless chemicals and useless chemicals into useful chemicals as much as possible. But these are not claims that you can easily substantiate with elementary students. Then again, I'm not opposed to just asserting “facts” without evidence on rare occasions, as “things we now know,” and if you do tell them that livers and kidneys help process chemicals and regulate nutrients, then you can point out something interesting about diet: Both the liver and the kidneys act like large “nutrient sponges”, and this makes both of these organs very nutritious to eat. As I understand it, wild carnivores will often eat the liver and the kidneys of their prey first. Modern nutritionists sometimes advise people to “eat more organ meats” because they are full of nutrients.]

A Map of the Entire Nutrition System — Now we have explored the entire “organ bundle” inside your torso, and we have figured out what most or all of the organs do. They all work together to take the food, water, and air that we put into our bodies and use these “raw materials” to nourish all the other parts of our bodies, and keep us healthy and active (and also to collect and dispose of leftovers and wastes). Just as your muscles and bones make your “machinery” or “musculoskeletal system,” and your sense organs and your brain make your “information and control system” or “nervous system,” so the entire bundle of organs in your torso make your “nutrition factory.” Your torso contains the most important components of your “circulatory system.” Let's draw a complete “traffic map” of all the substances that run through our bodies. Let's draw a map of the entire “nutritive system” or “circulatory system,” and that will complete our collection of maps of the human body.