606 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



there follows an amount and variety of response in the organism as a whole 

 out of all proportion to the strength of the physical stimuli employed. Owing 

 also to the wide connections of the nervous system and the conduction of all 

 incoming impulses to its central part a measure of harmony is maintained 

 between the various activities of the several systems composing the body. 

 Thus not only the various systems forming the body are in this manner con- 

 trolled, but the body as a whole, in relation to all things outside of it and 

 forming its environment, is even more plainly under the guidance of these 

 administrative cells. 



Growth and Organization. In this connection, it is fitting to emphasize 

 a character of the central system which is both unique and highly important. 

 The physiological connections existing between the nerve-elements in youth 

 are very incomplete and poorly established, more so than in any other system 

 of the body ; in the history of the growth of the nervous system, the increase 

 in weight and change in shape run parallel with an increase in its organiza- 

 tion i. e. in the connections between its constituent cells. This organization 

 results in better and more numerous physiological pathways which permit the 

 system, as a whole, not only to do more perfectly at maturity those things 

 which it could do in some degree at an earlier age, but also, by virtue of its 

 increased complexity, to do at maturity those things which previously it could 

 not do at all. 



Growth in the case of this system implies, therefore, an increase in com- 

 plexity such as nowhere else occurs, and since this growth can be modified by 

 the experience of the individual during the growing period, the importance 

 of understanding it and its relation to organization is evident. 



Phenomena Involving Consciousness. It is with the nervous system 

 that the phenomena of consciousness are most closely linked. Strictly, physi- 

 ology concerns itself at present with the reactions of the nervous system, which 

 can be studied without an appeal to consciousness. A moment's consideration 

 shows, however, that in the physiology of the brain the assistance to be 

 obtained by passing beyond the limit thus laid down is of more value than 

 any boundary, and hence, although the field of consciousness is sacred to psy- 

 chology, physiology should not be deprived of any of the advantages which 

 come from the privilege of occasional trespass. 



Plan of Presentation. In accordance with these facts, it has seemed best 

 to first present 



Part I. The physiology of the nerve-cell, considered as a peculiar kind of 

 tissue-element, endowed with special physiological characters. 



Part II. The activities of the simplest groups of these elements. The 

 physiological grouping is of course mainly dependent on the anatomical 

 arrangement, and, as must always be the case, the activities of one group 

 modify those of others. Stated in general terms, the problem in this part is 

 that of the pathway of any impulse through the central system. 



Part III. The reactions of the system taken as a whole. Here its capa- 

 bilities as a unit are contrasted with those of the other tissue-systems, and its 



