172 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



between the activities of the several systems composing the body. Thus, not 

 only the system.- forming the body are in this manner controlled, 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 administra- 

 tive cells. 



Phenomena Involving - Consciousness. — It is with the brain and espe- 

 cially the cerebral hemispheres that the phenomena of consciousness are most 

 closely linked. Strictly, physiology concerns itself, at present, with the reac- 

 tions of the aervous system which can he studied without an appeal to con- 

 sciousness. 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 psychology, physiology should not be deprived 

 of any of the advantages which come from the privilege of occasional 

 trespass. 



Growth and Organization. — The physiological connections existing be- 

 tween the nerve-elements in infancy are very incomplete and poorly estab- 

 lished, 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 complexity. This increase in complexity 

 i- accompanied by more perfect organization, which results in better and 

 more numerous physiological pathways, thus permitting 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 com- 

 plexity, 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 impor- 

 tance of understanding it and its relation to organization is evident. 



Plan of Presentation. — Our subject may be discussed under three heads 

 dealing respectively with the physiology of the (1) single cells, (2) groups of 

 • •ells, ami (•"> i the entire central system. 



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

 of tissue-element, endowed with special physiological characters. 



Part II. The activities of groups of these elements. The physiological 

 grouping is, of course, mainly dependent on the anatomical arrangement. At 

 the same time, the activities of one group modify those of others. Stated in 

 general terms, the problem in this pari is that of the pathway of any imj)ii/.<, 

 through tin- 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 systems, and its 

 growth, organization, and rhythms of resl and activity are most readily de- 

 scribed. 



