1899] NERVOUS SYSTEM IN ORGANIC EVOIUTION 255 



the power of reaction to the incident forces of the environment, and 

 ignoring the desires demanding satisfaction which arise de novo within 

 the brain itself. To a physician, on the other hand, the nervous system 

 is by far the most important part of the human body. He knows 

 that all medicines that act physiologically, and not purely chemically 

 or mechanically on the system, do so through the nervous system. He 

 knows experimentally that if the nerves to the organ on which a 

 medicine acts be severed, the organ fails to respond. We know that 

 if undue heat be applied to a portion of the surface of the body the 

 vessels dilate and the part becomes redder, because on the heat 

 being applied the terminal nerves telegraph to the nearest nerve- 

 centre that help is needed to resist the irritation. Through the 

 vaso-motor nerves controlling the calibre of the blood-vessels these 

 dilate, probably that the increase of blood may carry off the excess of 

 heat ; the part thus making an effort to ward off injury. We may 

 well assume that if the nervous connection were severed no dilatation 

 of blood-vessels would take place, and in consequence the parts would 

 suffer. Again, we know from the study of diseases that if the 

 centre in the spinal cord for the nutrition of any special muscle be 

 destroyed by inflammation, the muscle gradually dwindles from lack 

 of nutrition. The whole study of pathology teaches us how, if through 

 disease or accident defects are produced, they are remedied through 

 the nervous influences operating correlatively on adjacent cells and 

 tissues. Again, if a large blood-vessel be destroyed either accidentally 

 or iDtentionally for purposes of cure, the small blood-vessels supplying 

 the parts affected and anastomosing with those of adjacent parts 

 gradually enlarge and carry on the function of the destroyed large 

 vessel, — a fact which shows us how the distribution of blood may 

 gradually become modified through functional change in the process of 

 evolution of one species into another. In experiments on animals we 

 learn that, although normally certain cells have a definite function, 

 yet if the nerves governing those cells be severed, so that the connection 

 between the cells and the nerve-centre is destroyed, the function of the 

 cells ceases, and that if the centre for the nutrition of the cells be also 

 destroyed the cells will die. Whether this is a direct result or due 

 indirectly to the loss of nutrition has not yet been positively determined, 

 probably it is due to the latter. Hence we can positively assert that 

 the cells of the organism have no inherent power in themselves to 

 exercise their function, or even to maintain their vitality, but that the 

 nerve-centres through their connections with the cells supply that 

 power which manifests itself as the function, and even as the vitality of 

 the cells themselves. Thus my contention is supported, that if in the 

 germ-cell the germ-plasm is the most important part as the bearer of 

 the life functions, so in the finished organism the nervous system is 

 the bearer of the like processes, commanding and controlling all life 

 and function. 



