faso) Ss WERVOCS SYSTEM IN ORGANIC EVOLUTION 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 intentionally 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. 
