At least one other important duty the ear performs; it tells us in what position we are, and how our whole head moves or is moved. On the top of the saccule, in [Diagram 57, Fig. 4], there are shown three little loops which are called the semicircular canals. They are shown again more clearly by themselves in [Diagram 59].
Fig. 1 shows their position with regard to each other. It will be seen that two of them are vertical, with their loops forming a right angle with one another, and that the other is horizontal—in fact, that they lie in the three planes of space. Fig. 2 shows the structure of one of them; it has a swelling at one end (a), and a knob projecting into it where the nerve joins it (b). In Fig. 3 is shown a section through this knob, which gives the key to the use of these structures. A little head of cells projects from the wall of the canal into its lumen, and from these cells hairs bristle out into a dome-like covering of jelly, weighted, to prevent its moving too easily, with small particles of lime. Now, if you take up a round vessel full of liquid—say a bowl of gold-fish—and give it a twist round, you will notice that, though the bowl turns, the water inside does not; the fish remain in their old position. If there were a rod projecting from the side of the bowl, it would, of course, move with it, and if a fish came in its way would strike against it. This is the principle of the semicircular canal. For if we turn our head, the tube of the canal turns, passing over the liquid in it, which of course does not move, though it appears to flow in the opposite direction. The consequence is that the hairs on the side of the knob in the direction in which the head is being moved are pressed upon by the dome of jelly, which, as it floats in the liquid, tends to remain where it is. The nerves, stimulated in this way, inform the animal generally of the movement.
These little organs are very important to us, though we have our eyes to correct our ideas of position, and they are still more so to the fish, which dart and turn in the wide expanse of the ocean, and the birds and bats, which wheel about in the air. There are, however, some occasions when we do not feel inclined to bless them; for, inasmuch as they faithfully report every roll and plunge of a ship to a person on board, it is they which are mainly responsible for sea-sickness.
And now that we have seen how the body lies with regard to the external world; how it is efficiently protected from its surroundings; how it is placed in communication with them; and have briefly examined the organs by which it makes its chemical and physical investigations, looks out into space, and is kept aware of what is going on therein, we may return to the means whereby it responds as a whole to the stimuli thus reported—the central nervous system—and try to learn how the right response is brought about.
IV.
There is but one thing more to describe in the mechanism of the body—the connecting link between the last two sections. In the last we saw how the body receives stimuli from the external world; in the one before, that when these stimuli reach the central nervous canal it in turn stimulates the organs to perform such movements as circumstances require. What, therefore, remains to be described is the working of that canal by which these necessary movements are ordered and controlled.
Now, in speaking of reflex action a few pages back, we said that the nerves which bring in stimuli from the periphery distribute them about the neural canal to those cells whose activity, by sending out fresh stimuli to the muscles, produces the requisite movements. These motor cells, however, are not scattered about the spinal cord anyhow. They are collected into clusters, or nuclei, as they are sometimes called, and each cluster has special duties—i.e., a special organ to control. Thus, we say that there are in the central nervous system centres—a nervous centre to control the leg; another to work the diaphragm; another for the muscles of the ribs; more for the arm, hand, etc. And these centres are in communication with one another, so that they may not pull different ways.
In the first example of reflex action given in [Section II. of this essay], the sensation of a pin-prick was first conveyed to the centres controlling the limb injured, by whose activity it was drawn away from the danger. But the nerve which gave the warning which produced this elementary movement distributed the impression that something was wrong to the higher centres, so that the whole body was involved in protecting, doctoring, and avenging the outraged member; from which it would appear that the lower centres are under control of higher ones. And this is the case. If we may be allowed the metaphor, there are captains of tens, who are under the direction of captains of fifties, and the captains of fifties receive their orders from captains of hundreds. The nerve canal, the manner of whose formation as a simple tube is shown in Diagrams [5] and [42], has therefore different functions in different parts, and this to such an extent that considerable differentiation in bulk and structure is produced.
The neural canal may be roughly divided into two parts—a comparatively simple tube, running the greater part of the animal’s length, containing many centres from which nerves run to the organs they control; and a complicated bulbous enlargement at one end, with thickened walls, in which are the centres controlling those in the cord, and thereby managing not so much organs as the whole animal. The former is called the spinal cord, the latter the brain.