I.
Now comes the final problem. Protoplasm forms a structure always changing, always making good its waste by chemical action upon raw material, always capturing raw material or in search of it, always, when it exists in large quantities, and the labour is therefore divided between many cells, economically apportioning the work and the spoils. How is it that all the actions, chemical as well as physical, of a vast number of cells composing a large body are, no matter how complicated, always harmonious, and always with purpose directed to the advantage of the whole animal?
In the first essay in this book we discussed the phenomenon of life, and described briefly the chemical and physical peculiarities of protoplasm. These in the two succeeding essays we have gone into more fully; but there is one characteristic of that interesting substance which yet remains for us to examine in specialized cells, viz., its extreme readiness to respond to changes in its environment.
In [Essay I.] we saw that chemical agents, light, heat, electricity, etc.—had a definite effect upon protoplasm, and that, though they might influence different kinds in different ways, the effect was nevertheless invariable; in a word, the response of protoplasm to circumstances is automatic. But the most remarkable thing about this is that the response is not confined to the protoplasm actually affected, but is transmitted to that nearest to the part stimulated, and again passed on to that beyond, so that a wave of excitation passes through the whole mass, not stopping till it has reached the extreme confines of the cell. It may even pass beyond these and set up activity in neighbouring cells. The power of conductivity once grasped, it may easily be seen that certain cells, by specializing in this direction and adapting their shape to the needs of the body, might by throwing out long threads to reach distant parts set up an organic system of telegraphy.
The organs developed for the control of the body owe their origin to the outer layer. ([See Diagram 5.]) This was only to be expected. In [the second essay], in which we treated of the chemistry of the body, we, of course, touched upon all three layers from which the body is built up; but the one which chiefly occupied our attention was the innermost layer, which is so admirably arranged as a chemical laboratory. In [the third essay] we dealt chiefly with the middle layer, which both by its position and its bulk might have been guessed to be the foundation of most of the motor organs. Now that we have come to the organs of perception and transmission of impressions, it is only natural to expect that they should be specialized from the cells already in contact with the external world, and which, since they form the envelope of the animal, must allow all such stimuli as reach the subjacent motor layer to pass through them.
Hitherto we have not dealt at great length with the development of the organs whose functions we have been describing, either from the point of view of the embryologist or the evolutionist. Nor have we spent much time upon their gross anatomy. With the nervous system we must proceed rather differently; for to understand how its higher functions can be performed they must be traced from their origin step by step, while their complexity is largely vested in the structure of special organs.
The way in which the nervous system was evolved is shown in [Diagram 5]. Originally, no doubt, the cells of the outer layer, when the latter was in its simplest form—that is to say, only one cell thick, not several, as it is in our skin—would, when influenced in any way directly call forth the activity of the motor cells lying beneath them. ([See Diagram 40, Fig. 1.]) In [Fig. 2], however, we see one cell of the outer layer becoming specialized. It has thrown out a process above the surface of the skin the more readily to catch impressions, and has sent another down into the body the better to distribute them. [Diagram 41, Fig. 1] shows the nerve cell at a further stage. The principle is the same, but the cell is removed to a safer place. In [Fig. 2] it is not exposed to the outside world at all, but by receiving its impulses second-hand from several cells the same work is done with greater economy and uniformity. Some of the special sense organs are still developed in this way.
Diagram 40.—Showing Origin of a Nerve Cell.
Diagram 41.—Showing the Development of a Nerve Cell.
Once the nerve cell is developed and safely shifted into the interior of the body, it is clothed with a protecting feltwork of connective tissue, and the nerve fibres are also surrounded by connective-tissue cells which secrete around them the fatty substance which makes nerves look white.
Such is the nerve cell or intermediary between the world and the muscles; but thence to harmonious movement in a body with complex organs capable of varied actions is a long step. To obtain precision and uniformity throughout the body, all the impressions received must be collected and balanced, and stimuli, the correct outcome of this balancing, must be transmitted to the muscles, glands, etc., whose activity circumstances require. The way in which cells of the outer layer become enclosed to form a central nervous system is shown in [Diagram 5]; but its development will be better seen in the figures of [Diagram 42].
Diagram 42.—To illustrate the Development of the Nervous System.
Diagram 43.—Cross-section of the Spinal Cord, showing how it gives off Nerves.
Diagram 44.
This diagram shows how certain cells of the outer layer are budded off and transferred to a safe place within the body. In this position the cells are further developed, throwing out one long fibre, which goes to some distant organ of the body, and short fibres, which, though they do not join those of other cells and become continuous, closely interlace and put them into communication. They are also separated from one another by connective tissue, which supports them, holding them suspended with only their fibres approaching one another ([Diagram 43]). [Diagram 44] shows how the bone which replaces the supporting rod (see [Diagram 6]) throws an arch round the feltwork of connective tissue in which the nerve cells are suspended, giving them still further protection.
It will be noticed in the figures of [Diagram 42], which is fuller than [Diagram 5], that there are three of these buds—one central and two lateral. The central one becomes a tube running the whole length of the animal, while the lateral buds form solid clusters or ganglia, arranged in pairs at intervals beside it ([see Diagram 45]). Fibres from these ganglia go to the skin, and bring to the nerve cells information from the outside world, which they duly pass on to the cells of the central column. The cells of the central column, when set in motion by the ganglion cells, send out impulses to the muscles, whose contraction is necessary to perform the movement which circumstances indicate. A movement brought about in this way is called reflex.
Diagram 45.—Central Nerve Tube and Ganglia.
The reflex movements are, however, not quite the simplest. For instance, the food is moved along the alimentary canal by the contraction of two sets of muscle fibres—an outer longitudinal coat and an inner circular one. Between these two coats are some nerve cells, which are thrown into activity by the presence of food and the iron compounds of the bile secreted by the liver in the tube. These sympathetic cells do not send their impulses to any centre for examination, but at once stimulate the muscle fibres between which they lie, thereby producing the peristaltic movements we have already described. Yet it should be remembered that, though these cells act independently of the central nervous system, they are under its control, and can, if need be, have their action modified for the benefit of the body as a whole.
For convenience’ sake, we had better here specify the chief kinds of nervous action. First there is what we may call the immediate nerve action, such as that we have just been describing; secondly there is reflex action, the centres for which are in the spinal cord and the base of the brain; and thirdly there is voluntary movement, which arises out of the interaction of centres in the hemisphere of the brain, where the most complex machinery of all is kept.