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

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

or swallow or walk about—indeed it spends nearly the whole of its time in prowling restlessly and aimlessly around—and at night it sleeps soundly. But these decerebrate animals differ from normal animals in the complete lack of any sign of psychical phenomena. Mentally they are dead. All intimations of feeling cease. They seem unable to remember or to anticipate. If food is put into their mouths they will chew it, swallow it, and digest it, but they never show any signs of appetite or hunger ; even when they are starving they never look for food, and if food is put into their cages they will not touch it of their own accord. They do not recognize it as food. And similarly with painful stimuli; a decerebrate frog will jump away if it is pinched, and a decerebrate dog will snarl and snap, but neither shows any sign of fear at a threat. They make no response at all unless they are directly stimulated. Perhaps the most noteworthy point about these animals is not the things they cannot do but the things they can do. Consider simply the fact that without cerebral hemispheres a mammal can walk about and avoid obstacles. Walking is a very complicated activity indeed. It involves the contraction of a

number of muscles each at the Vertebrae

right moment—moreover, when any muscle contracts those which oppose its action relax their tone ; it involves a stream of sensory impulses from the limbs that report progress so that the central nervous system may know what to do next. Butit also involves other things, for the organism

must keep its balance. There are Fig. 53.

impulses streaming in from the

labyrinth of the ear, from the

pressure organs in the soles of the feet, from the eyes, which tell the central nervous system about the position of the body ; and there are always slight movements of the body to be made, movements which are not propulsive but which keep the body balanced while it is propelled. Clearly there are hundreds of different reflexes going on, all of which are perfectly automatic. In the intact mammal—ourselves, for examplethere is always a similar undercurrent of reflex activity. When we stand erect, or even when we sit up in a chair, there is a constant stream of impulses from the senseorgans responsible for our equilibrium, and a constant counter-stream that controls the

tautness of our muscles and makes slight adjustments of the positions of our parts. And in this category come many of our other activities—such as breathing movements ; but we cannot catalogue them here. Neither, unhappily, can we go into the mass of accurate information that has been collected about reflex actions. Roughly speaking, they depend on the way in which nerve-fibres are disposed in the central nervous system ; they are literally parts of our anatomy. But the story is a complicated one, and it still has its obscure passages. We shall return to it in a later book. All we can do here is to remind Mr. Everyman

Cerebrum

Nerves

The brain and upper end of the spinal cord exposed from the right.

that the most humdrum activities of his daily life depend for their due performance upon the existence of a piece of machinery whose construction of living threads and_ cells far outdoes in complication any machine of metal or rubber or glass ever constructed by men. ;

The cerebrum, the organ of the mind, is superposed upon this system of automatic, machine-like responses, and it is capable of interfering with and modifying the activity of the reflex centres. Returning to our comparison of the body to a state and the central nervous system to a government, the spinal cord and the brain-stem may be compared to clerical and administrative