186 life's beginning on the earth 



became more and more exaggerated. Finally, it was cabled 

 to Europe in a form that produced the impression that 

 Dr. Crile had succeeded in making new life without pro- 

 genitors, in the laboratory. 



7. NERVES AND MUSCLES 



In spite of all our efforts, we have accomplished no more 

 than a partial understanding of the movements of the most 

 primitive organisms. Hardly anything is understood con- 

 cerning the mechanism of the muscles by which the more 

 highly developed organisms move. The muscle is, more- 

 over, no independent organ of motion; it can only work if 

 connected with a nerve from which it receives a stimulus, 

 which causes it to contract. It cannot even exist without 

 a nerve, since it fades away if its nerve is severed. 



A muscle consists of thousands of fibers and each fiber in 

 turn is a bundle of still smaller fibers, or fibrils. Each 

 nerve consists of several thousand nerve-fibers, just as a 

 cable consists of a bundle of single wires. A microscopic 

 view of a muscle and of a nerve-fiber is shown in Figure 74. 

 All nerve-fibers ending in muscles form a part of an in- 

 tricately interwoven system of countless other nerve-fibers, 

 all of which terminate in the brain. The brain itself may 

 be looked upon as a most highly-involved network of bil- 

 lions of nerve fibers, which form billions or trillions of inter- 

 connecting pathways. From this inextricable maze there 

 emerge other fibers which terminate in the skin, or in the 

 sense organs such as the eye, ear, nose, and mouth. These 

 fibers carry sensations from the skin first to the brain. 



A multitude of pathways thus exists for the propagation 

 of all those impulses which arise by sensation of touch, pain, 

 heat or cold, or through the eye, ear or other sense organs. 

 These impulses pass through the fiber net-work of the brain 

 or spinal cord, and terminate in muscles, glands, or other 

 organs which they excite. 



The nervous system acts not only upon the muscles which 



