This division, accustomed as we all are to take it for granted, offers plenty of food for reflection. Why should an animal have such a brain placed in its head? Why, indeed, should it have a head, regarding that member as a group composed of eyes, nose, mouth, ears and brain? The mouth gives us the key to the riddle; the mouth is the essential organ, and all the rest are its accessories.
In [the first essay] we saw that the basis of life was chemical, and in the second that the materials necessary for the chemical action, or food, must, in the higher animals, be taken into the digestive tube through the opening which we call the mouth. Therefore, as it is highly important that only the most beneficial substances shall be received into it, and that all which are actively injurious shall be excluded, it is plain that the organs of chemical perception must be placed in its neighbourhood—the organs of smell to enable the mouth to find its food, and the organs of taste to aid in selecting it. As, moreover, our humble ancestors, the fishes, move literally mouth foremost, it is not surprising to find the organs of space perception, the eyes, also situated in its neighbourhood, especially when one considers that their food is often of a lively character, and requires precision of movement to secure it. The inevitable consequence of thus grouping the more important organs of perception under the fore-end of the neural canal is that it grows and develops more highly here than elsewhere along its length, and soon is in a position to dictate to the rest of the body. Another reason why it must develop is that it must contain centres for turning its impressions to practical account, not only by producing complicated movements in the jaws, eyes and gills, but also by ruling the centres in the cord, and instructing the body to carry the mouth whither it needs to go.
Diagram 60.—Showing Primary Division of Nervous Tube.
In the preceding diagram ([60]) the origin of the brain is shown as a dilatation of the end of the neural canal into a bulb with thickened walls, which has already become constricted in places, so that it is subdivided into three. The next diagram ([61]) is intended to give, in no matter how crude and schematic a way, some idea of the lines on which the development continues. We do not show all, or even half, the structures which go to make up the brain. To do so would be out of place in a book like this. Further, we shall endeavour as far as possible to speak of the brain in general terms, avoiding the five-syllable bastard Græco-Latin names with which the early anatomists have endowed almost every square inch of its substance, and confine ourselves to summing up its functions as briefly as can be done with justice.
Diagram 61.—Giving a Rough Idea of how the Brain is developed.
In pursuance of this method, attention must be drawn to the fact that only the foremost of the three original bulbs (marked A in the diagram) and the hindermost (C) continue to grow. The middle one (B) remains comparatively simple. From the foremost lobe buds grow out to form the eyes in the manner which we have already described, and other buds push forwards to meet the nerves from the nose. The latter have, even in the early stages shown in [Diagram 61], reached an extraordinary size; and when we come to trace them further, we shall find that they become very complex, and acquire remarkable and unexpected powers, considering their humble origin. Strange changes also take place in the hindermost bulb. It splits along the top, so that the cavity it contains is open like a saucer, though bridged over by a three-lobed body called the cerebellum.
Following the spinal cord up into the brain, we are conscious of no sudden line of demarcation separating the one from the other, only of an increasing size and complexity. The lower parts of the brain send out and receive nerves much as the cord does; three pairs go to the muscles which turn the eyes; other pairs bring in sensations from the face and throat; others control the muscles of the face, tongue and throat. But the brain differs from the cord in being directly connected by nerves, not only with adjacent parts, but also with the distant and more important organs in the interior of the body—heart, lungs, etc.; in containing groups of cells which have stimuli sent on to them from all over the body viâ the cord; and in possessing centres which control those lower down in the nervous system. It therefore not only receives and balances stimuli from all over the body, but, by governing the centres which preside over the bodily movements, is able to wield and direct the body as a whole.