TEE NERVOUS SYSTEM 125 



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cerned, the state met with in the muscles of the sponges. These ex- 

 amples then show that even in the higher animals certain muscles 

 respond normally to direct stimulation and thus exhibit a form of 

 activity which is believed to be generally characteristic of sponges. 



In my opinion the simultaneous origin of nerve and muscle can no 

 longer be maintained. Muscle arose first and the simple effectors thus 

 produced were the first element of the neuromuscular mechanism. 

 These effectors were directly stimulated and consequently slow in ac- 

 tion. They afforded centers around which nervous tissue first differen- 

 tiated in the form of sense organs or receptors whose function it was to 

 serve as triggers to initiate muscle action quickly. As these receptors 

 became more highly developed, a third element, the central nervous 

 organ, arose from the nervous elements between the receptor and the 

 effector. This organ, the adjuster, served as a means of conducting and 

 modifying the sensory impulses on their way from the receptor to the 

 effector and ultimately it also served as a storehouse for the nervous ex- 

 perience of the individual and as the seat of its intellectual life. It is 

 interesting to observe that this view of the origin of the nervous system 

 is in accord with the philosophical speculations of Bergson according to 

 whom the nervous system has been evolved primarily as an organ for 

 animal response and only secondarily as one concerned with intellectual 

 activities. 



But if we picture the nervous system as having arisen as an append- 

 age to the musculature and as having grown in complication as the 

 musculature became differentiated, we are still far from an adequate 

 view of even the more obvious aspects of its evolution. The nervous 

 system controls many more kinds of effectors than muscle and its sen- 

 sory elements are vastly more complex than is implied in the preceding 

 sketch. To gain a more comprehensive view of the evolution of these 

 organs, it is necessary to consider a subsidiary but important process, 

 the appropriation of effectors and receptors. 



The nervous system of many of the higher animals not only acts 

 upon the musculature; it may also control electric organs, luminous 

 organs, chromatophores, glands, etc. Not all such organs are under 

 the influence of the nervous system, but it is not difficult to find for 

 each group of effectors animals in which the given type of organs is 

 under the influence of nerves. The electric organs and the chromato- 

 phores of fishes, are of this kind as well as the salivary glands of the 

 mammals and the luminous organs of the brittle stars. 



How has the nervous system gained control over these organs? 

 Except the electric organs, which are probably modified muscle, all these 

 organs have arisen in my opinion as independent effectors. Most of 

 them can be identified as such in one group of animals or another. 

 Thus among the glands the pancreas in the higher vertebrates has been 



