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

BOOK 1

muscles (such as those of the arms and legs), and some are serial (such as those of the

Palate Spinal Cord

Tongue Gullet Windpipe

Breast Bone

Fig. 5. A diagram of the human body seen from the side, to show as simply as possible how the more important organs are disposed.

ribs), the actual number of muscles in the human body is very much greater. But let us leave to the medical student the labours peculiar to his profession. These hundreds of muscles that move the human machine about are controlled by the nerves, tough white cords of tissue, some thick and conspicuous, some so thin as to be barely visible, running and branching among the quivering muscles. As everyone knows, they function as telephone wires ; they bring stimuli to the muscles whenever the latter are to move. Their arrangement is quite as complex and intricate as that of the muscular system, for each muscle must have its own nerve-supply. They radiate out from the brain and its continuation the spinal cord. Brain and spinal cord constitute a government, so to speak, which rules the muscles; it is constantly flashing instructions along the nerves and commanding a movement here and a relaxation there. Without this central control—if, for example, its nerve is

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THE SCIENCE OF LIFE

Fig. 6.

CHAPTER 2

cut or diseased—a muscle is paralysed, flabby, and helpless.

The brain and spinal cord do not exercise a wanton or capricious dictatorship ; besides sending commands they receive a copious stream of information from all the parts of the body, and laboriously respond to that stream and move the mechanism in appropriate ways. ‘The central government is the brain. Its informants are the sense-organs —eyes, ears, nose, tongue, the organs of touch, warmth and pain, the organs in our muscles and joints that feel at any moment what our limbs are doing, the others in our chests and abdomens. These and many more are continually sending in their messages to the brain and the spinal cord, and the incoming stream, like the outgoing stream of instructions, is also borne by nerves. As our imaginary professor told us at the outset, the brain-case is the upper part of the head and the nose and mouth the lower. In a man, having a powerful brain and feeble teeth, the former part is by far the larger ; the brain-case overshadows the face. In a mouse, with a comparatively ill-developed brain but with large and wonderfully specialized teeth, the brain-case is the smaller of the two.

The blood-vessels that supply our muscles are of two kinds—the thick elastic arteries, along which the warm pulsing stream of blood is pumped, and the thinner veins, along which it flows sluggishly back. Between the two there is a third system of very fine vessels, for the arteries lead into ja

Humerus

UlIna The movements of the limbs are produced by the pull of shortening muscles on the jointed bones.

The figure shows how the biceps, attached to the shoulder and . forearm, produces a bending at the elbow joint.

network of invisibly small tubes, the capillaries, from which the veins lead away. ‘The