Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (13 page)

What made me so hard-core is that head anatomy is deeply mesmerizing, in fact, beautiful. One of the joys of science is that, on occasion, we see a pattern that reveals the order in what initially seems chaotic. A jumble becomes part of a simple plan, and you feel you are seeing right through something to find its essence. This chapter is about seeing that essence inside our own heads. And, of course, the heads of fish.

THE INNER CHAOS OF THE HEAD

Head anatomy is not only complicated but hard to see, since, unlike other parts of the body, the tissues of the head are encapsulated in a bony box. We literally have to saw through the cheek, forehead, and cranium to see the vessels and organs. Having thus opened a human head, we find a clump of what looks like tangled fishing lines. Vessels and nerves make curious loops and turns as they travel through the skull. Thousands of nerve branches, muscles, and bones sit within this small box. At first glance, the whole array is a bewildering mess.

Our skulls are made up of three fundamental parts: think plates, blocks, and rods. The plates cover our brain. Pat the top of your head, and you are feeling them. These large plates fit together like jigsaw-puzzle pieces and form much of our cranium. When we were born, the plates were separate; the open spaces between them, the fontanelles, are visible in infants, occasionally throbbing with the brain tissue underneath. As we grow, the bones enlarge, and by the time we reach the age of two they have fused.

Another part of our skull lies underneath the brain, forming a platform that holds it up. Unlike the plate-like bones at the top, these bones look like complicated blocks and have many arteries and nerves running through them. The third kind of bone makes up our jaws, some bones in our ears, and other bones in our throats; these bones start development looking like rods, which ultimately break up and change shape to help us chew, swallow, and hear.

Inside the skull are a number of compartments and spaces that house different organs. Obviously, the brain occupies the largest of these. Other spaces contain our eyes, parts of our ears, and our nasal structures. Much of the challenge in understanding head anatomy comes from seeing these different spaces and organs in three dimensions.

Attached to the bones and organs in the head are the muscles we use to bite, to talk, and to move our eyes and whole head. Twelve nerves supply these muscles, each exiting the brain to travel to a different region inside our head. These are the dreaded cranial nerves.

Plates, blocks, and rods: the theme for skulls. Every bone in our head can be traced to one of these things.

The key to unlocking the basics of the head is to see the cranial nerves as more than a jumble. Indeed, most of them really are simple. The simplest cranial nerves have only one function, and they attach to one muscle or organ. The cranial nerve that goes to our nasal structures, the olfactory, has one job: to take information from our nasal tissues to our brain. Some of the nerves that go to our eyes and ears are also simple in this way: the optic nerve is involved with vision; the acoustic nerve works in hearing. About four other cranial nerves only serve muscles—working to move the eyes inside the orbits, for example, or to move the head around on the neck.

But four of the cranial nerves have given medical students fits for decades. For good reason: the four have very complex functions and take tortuous paths through the head to do their jobs. The trigeminal nerve and the facial nerve deserve special mention. Both exit the brain and break up into a bewildering network of branches. Much like a cable that can carry television, Internet, and voice information, a single branch of the trigeminal or facial nerve can carry information about both sensation and action. Individual fibers for sensation and action emanate from different parts of the brain, are consolidated in cables (what we end up calling the trigeminal and facial nerves), then break up again, sending branches all over the head.

The trigeminal’s branches do two major things: they control muscles, and they carry sensory information from much of our face back to our brain. The muscles controlled by the trigeminal nerve include those we use to chew as well as tiny muscles deep inside the ear. The trigeminal is also the major nerve for sensation in the face. The reason a slap to the face hurts so much, beyond the emotional pain, is because the trigeminal carries sensory information from the skin of our face back to our brain. Your dentist also knows the branches of your trigeminal nerve well. Different branches go to the roots of our teeth; a single jab of anesthetic along one of these branches can deaden the sensation of different parts of our tooth row.

The facial nerve also controls muscles and relays sensory information. As its name implies, it is the main nerve that controls the muscles of facial expression. We use these tiny muscles to smile, to frown, to raise and lower our eyebrows, to flare our nostrils, and so on. They have wonderfully evocative names. One of the major muscles that we use in frowning—it moves the corners of our mouth down—is called the depressor anguli oris. Another great name belongs to the muscle we use to furrow our brow in concern: the corrugator supercilli. Flare your nostrils and you are using your nasalis. Each of these muscles, like every other muscle of facial expression, is controlled by branches of the facial nerve. Things like an uneven smile or asymmetrically drooping eyelids are a sign that something might be wrong with the facial nerve on one side of a person’s face.

You are probably beginning to see why I was staying up so late to study these nerves. Nothing about them seems to make any sense. For example, both the trigeminal and the facial nerves send tiny branches to muscles inside our ears. Why do two different nerves, which innervate entirely different parts of the face and jaw, send branches to ear muscles that lie adjacent to one another? Even more confusing, the trigeminal and facial almost crisscross as they send branches to our face and jaw. Why? With such oddly redundant functions and tortuous paths, there seems to be no rhyme or reason to their structure, much less to how these nerves match up with the plates, blocks, and rods that make up our skull.

In thinking about these nerves, I am reminded of my first days here in Chicago in 2001. I had been given space for a research laboratory in a hundred-year-old building and the lab needed new utility cables, plumbing, and air handling. I remember the day when the contractors first opened the walls to get access to the innards of the building. Their reaction to the plumbing and wiring inside my wall was almost exactly like mine when I opened the human head and saw the trigeminal and facial nerves for the first time. The wires, cables, and pipes inside the walls were a jumble. Nobody in his right mind would have designed a building from scratch this way, with cables and pipes taking bizarre loops and turns throughout the building.

And that’s exactly the point. My building was constructed in 1896, and the utilities reflect an old design that has been jerry-rigged further with each renovation. If you want to understand the wiring and plumbing in my building, you have to understand its history, how it was renovated for each new generation of scientists. My head has a long history also, and that history explains complicated nerves like the trigeminal and the facial.

For us, that history begins with a fertilized egg.

THE ESSENCE IN EMBRYOS

Nobody starts life with a head: sperm and egg come together to make a single cell. Between the moment of conception and the third week thereafter, we go from that single cell to a ball of cells, then to a Frisbee-shaped collection of cells, then to something that looks vaguely like a tube and includes different kinds of tissues. Between the twenty-third and twenty-eighth days after conception, the front end of the tube thickens and folds over the body, so the embryo looks as if it’s already curled up in the fetal position. The head at this stage looks like a big glob. The base of this glob holds the key to much of the basic organization of our heads.

Four little swellings develop around the area that will become the throat. At about three weeks we see the first two; the other two emerge about four days later. Each swelling looks quite humble on the outside: a simple blob, separated from the next by a little crease. When you follow what happens to the blobs and creases, you begin to see the order and beauty of the head, including the trigeminal and facial nerves.

Of the cells inside each blob, known as arches, some will form bone tissue and others muscle and blood vessels. There is a complex mix of cells inside each arch; some cells divided right there while others migrated a long way to enter the arch itself. When we identify the cells in each arch according to where they end up in the adult, things start to make a lot of sense.

Ultimately, the first arch tissues form the upper and lower jaws, two tiny ear bones (the malleus and incus), and all the vessels and muscles that supply them. The second arch forms the third small ear bone (the stapes), a tiny throat bone, and most of the muscles that control facial expression. The third arch forms bones, muscles, and nerves deeper in the throat; we use these to swallow. Finally, the fourth arch forms the deepest parts of our throat, including parts of our larynx and the muscles and vessels that surround it and help it function.

If you were to shrink yourself to the size of a pinhead and travel inside the mouth of the developing embryo, you would see indentations that correspond to each swelling. There are four of these indentations. And, like the arches on the outside, cells on the indentations form important structures. The first elongates to form our Eustachian tube and some structures in the ear. The second forms the cavity that holds our tonsils. The third and fourth form important glands, including the parathyroid, thymus, and thyroid.

What I’ve just given you is one of the big tricks for understanding the most complicated cranial nerves and large portions of the head. When you think trigeminal nerve, think first arch. Facial nerve, second arch. The reason the trigeminal nerve goes to both the jaws and the ear is that all the structures it supplies originally developed in the first arch. The same thing is true for the facial nerve and the second arch. What do the muscles of facial expression have in common with the muscles in the ear that the facial nerve supplies? They are all second arch derivatives. As for the nerves of the third and fourth arches, their complex paths all relate to the fact that they innervate structures that arose from their respective arches. Those third and fourth arch nerves, among them the glossopharyngeal and vagus, follow the same pattern as the ones in front, each going to structures that developed from the arch they are associated with.