CHAPTER XV 

 NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 



THE necessity of means of bringing into relation with each other the different 

 parts of an organism, as it grows in size and complexity, has been already alluded 

 to (page 378). Moreover, the same muscles may require to be put into action, 

 say for flight, when an animal either sees an enemy or is touched by one, or for 

 the obtaining of food. If there were nerve channels directly connecting every 

 sense organ with every muscular group, the multiplicity of communications would 

 be most wasteful, besides inefficient for its purpose. The comparison of the 

 central nervous system to a telephone exchange is often made, and is quite 

 appropriate. Any subscriber can be put into communication with any other. 

 Similarly, in the animal, impulses arriving from a particular source may be, 

 according to circumstances, connected up, as it were, to different muscular 

 mechanisms. It will be seen that, since the arrangements of a telephone exchange 

 are mainly a matter of wiring, so, in great part, the study of nerve centres consists 

 of the anatomy of tracts of nerve fibres ; a study which tells us of the possible 

 ways of communication between the centres controlling various parts. This 

 aspect of the subject can best be studied in textbooks devoted to it, and will only 

 be treated incidentally in this book. An account, in some detail, will be found 

 in Starling's book (1912, pp. 324-632). 



There is another aspect, which is of a more general nature, and consists in 

 the investigation of the means by which the functional use of the paths provided 

 is arranged for. 



The co-ordination of the activities of various parts of the organism, or 

 integration, as the essential function of the central nervous system, is especially 

 insisted on by Sherrington (1906). When we consider that in the organism, 

 just as in the community, progress depends on the most effective working together 

 of the component units for the common good, we see how, as Gaskell has shown 

 (1908), the nervous system has been the dominant factor in evolution. Other 

 systems have been modified and changed in function in order to give opportunity 

 for the growth of this pre-eminently important one. As Gaskell puts it (p. 19), 

 " The law of progress is this The race is not to the swift, nor to the strong, but 

 to the wise." 



ORIGIN OF THE NERVOUS SYSTEM 



Different views have been put forward with regard to the way in which nerve 

 centres first made their appearance. A brief account will be found in the paper 

 by G. H. Parker (1911). The most satisfactory hypothesis seems to be that of 

 this investigator, who finds that sponges, although no nervous structures are to be 

 found in them, and although they exhibit none of the characteristic rapid reactions 

 of animals with even the most primitive nervous system, do nevertheless show 

 contractile response to stimuli. The opening and closing of oscula takes place in 

 response to movements of the sea water and is brought about by contractile tissue, 

 similar in its appearance and slowness of response to smooth muscle in the higher 

 animals (see Parker's paper, 1910). 



Muscular tissue makes its appearance, then, before nervous tissue. Even if we 

 regard the sponges as arising from protozoa by a branch which is separate from 

 that taken by the Coelenterates, it must be admitted that their organisation is a 



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