The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

whiskers. By making it captive we can spy upon its shy existence. It balances between fear and hunger and hunger and fear. Yet also there is something else in its life that can at times prevail over either of these forces. It responds to something, we may call it an obligation to the species; storms of desire impel it to do strange and dangerous things _so that the race may continue.

Here, then, we have three imperatives that govern the life of the mouse and to which its conduct must conform. It must fill its belly, it must save its skin, and it must breed ; and the penalty for failure is death, either of the sinner or of his stock.

So with any wild mammal that we choose to name, and so, perhaps to a lesser degree, with ourselves. All normally are under

Fig. 37. A nerve cut across and examined under the microscope as seen to consist of an enormous number of fibres lying side by side.

The area shown was, in the actual nerve, about one-fortieth of an inch across.”

these three imperatives. This is why the brotherhood of viscera—heart, stomach, kidneys, lungs—needs_ other helpers ; muscles to move them about, eyes, ears, and so forth to observe their surroundings, a nervous system to govern the community and hold it together. A living body is a co-operative alliance between a system of chest and belly viscera on the one hand and a system of behaviour organs on the other. ‘The former provide the latter with a stream of clean, nourishing blood—that is their essential duty in the partnership ; while the latter protect and guard the former and keep the stomach filled. We have studied the former group; we will turn now to the organization of watchful sense-organs and telephone nerves, by which the community is guarded and held together.

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THE SCIENCE OF LIFE

CHAPTER 3

§ 4 The Controlling System

Everyone has a general idea what the brain and spinal cord are like and how they are situated in skull and backbone. From them come nerves, spreading and branching out from the brain and passing through bony apertures in the cranium, not only to the face and head generally, but sending branches down even to the neck and heart ; and also from the spinal cord, in pairs, one pair between every vertebra and its neighbour, to the limbs and body. We have already compared the nervous system to a telephone system—from which it differs in the fact that its fibres are one-way fibres that either carry outgoing impulses (motor fibres) or bear incoming ones (sensory fibres). There are in addition to this main system (Central Nervous System) subordinate centres and systems in more or less complete communication with it. They perform tasks of coordination of a lesser scope, with whose details it is unnecessary to burthen the main system. The solar plexus, a network of cells and fibres concerned with abdominal activities, is chief among these inferior centres.

Now the structure of the brain and most of its relationship to the spinal cord we shall find it better to defer to a later part of the work. Upon it centre some of the most fascinating, illuminating, and controversial questions concerning life in general, and human life in particular, that it is possible to raise. In man and mouse the Central Nervous System has a close general resemblance, but the brain of the man is far larger, more powerful, and intricate in its working. ‘There centres the behaviour of the creature as a whole. Whatever is least mechanical, and anything there may be that is not mechanical in life, have their seat there. ‘There it is we must face the problems of consciousness and the freedom of initiative. ‘To understand it as far as current knowledge goes will be to crown and complete the task we are setting ourselves in this compilation, and at present we are only opening up the introductory matter of our subject. Here we will consider the nervous system and its working as the machinery of communication