The commonwealth of cells

by H. G. F. Spurrell

The hinder divisions of the brain, which we shall consider first, have no connection with consciousness or volition. They only produce reflex movements, which, however, owing to the wealth of material they have to work upon, are wonderfully complex and far-reaching.

Let us take a few examples. In the hindermost division of the brain (C in the diagrams) there is the centre which presides over the oxygen supply, the importance of which we saw in [the essay on vital chemistry]. This centre perceives when the lungs have been filled with a gas, and causes them to be emptied; it perceives when they are empty, and again does not allow them to remain too long in that state, before ordering an inspiration; it notes the quality of the air which is passing through the nose, and it notes the quality of the blood which bathes its own cells. The condition of the blood, indeed, is closely watched. An excessive quantity of carbonic acid gas, poverty of oxygen, even temperature, all produce through it an effect upon the rhythm of the breathing.

Close by the respiratory centre is the centre which controls the circulation. But enough has been said in [the section on reflex action], wherein the process of fainting was described, to give an idea of the part it plays in the body; so it need not detain us here.

We cannot, however, pass over its neighbour, the centre of temperature, so briefly. Its methods not only afford one of the most striking and interesting examples of harmonious regulations by reflex action, but the subject of temperature itself is so important that we must describe in some detail how that of the body is kept level.

As we said when discussing protoplasm generally, life—that is, the change always going on in the protoplasmic substance—is influenced by temperature: the single cell becomes less active at a low temperature, and dies at a high one; so obviously there is a temperature at which its functions are most easily carried on. Inside the body the cells are all kept at the temperature best for them by the circulation of the blood; but the absolute temperature of the whole body depends upon the heat which is generated within it by chemical action, and the heat which it loses to, or receives from, its surroundings. Under normal conditions this temperature in man is 98·4° F., when the production of heat from its own metabolism is balanced by the loss of heat by radiation. If, however, the atmosphere be very hot, less heat is developed in the body, the general metabolism being slower; and more heat is lost, since by reflex action the skin is bathed in sweat and cooled by its evaporation, and the small bloodvessels under the skin are dilated, so that more blood being brought to the surface, its chance of being cooled by radiation is thereby increased. If, on the other hand, the atmosphere is cool, the loss at the surface is minimized by constriction of the cutaneous bloodvessels, and a checking of the perspiration and consequent evaporation; while internally more heat is generated by increased metabolism. The cells which are mainly responsible for the production of heat are those of the muscles; and when much heat is required they increase in activity, not only in their general tone, but even by a visible movement, which we describe as shivering. So, within reasonable limits, whatever the temperature of its surroundings may be, that of the body remains the same, and though we may raise or lower our temperature by lying in a hot or cold bath, reflex adjustment of the sweat glands, bloodvessels and muscles brings it quickly back to normal when we emerge.

With a passing mention of the cerebellum, the three-lobed organ shown in [Diagram 61], and seen again in a more advanced stage in [Diagram 63], we may dismiss the two hinder divisions of the brain.

The cerebellum lies on the upward path of fibres from the cord to the higher centres in the fore-brain. It is a somewhat complicated organ, and its functions are not yet fully known. The older physiologists took a very extreme view of its importance, assigning to it, among other romantic duties, that of providing a habitation for the soul. This opinion on the strength of later research we can hardly endorse. The cerebellum really seems mainly concerned in co-ordinating the action of the muscles, especially in maintaining equilibrium in standing and walking.

Our knowledge of the whole brain is very far from complete. We should like to know the peculiar function of each little group of cells that can be made out under the microscope, and the paths of all the fibres connecting the different parts of the nervous system. As it is, we have to wait with the best patience we can while they are being investigated, and hope. In few departments, however, have the labours of the physiologist proved more fruitful and interesting than in the study of the fore-brain (A in the diagrams).

In the simpler form, as shown in [Diagram 61, A], and [Diagram 62, Fig. 1, A], the fore-brain is remarkable in that it throws out buds for the two most important sense organs—those of sight and smell. So important are these senses, especially in our humble ancestors, as we have already pointed out, that it is not surprising to find the impressions of the other senses brought on up from the hinder parts of the brain to be compared with them. The fore-brain is, in fact, a sort of terminus whither the whole of the afferent or incoming stimuli are brought, and whence, since information is only received in order to be acted upon, the supreme orders to the body issue.