CHAP. II.] A NERVOUS SYSTEM. 29 



nerve tissues of younger animals than of those in whom 

 the nerve centres are more fully elaborated. The chemical 

 compounds, entering into the constitution of nerve tissues, 

 are also extremely complex and very unstable. Thus, both 

 from their physical and chemical composition, it is thought 

 that waves of molecular movement are easily initiated in 

 and easily propagated through nerve cells and fibres. 

 Whether these molecular ' waves ' or ' currents ' in nerve 

 tissue are brought about by virtue of mere isomeric 

 changes or by actual decompositions occurring in their 

 substance is, for the present, extremely doubtful.* 



Our knowledge of the exact arrangement of the ana- 

 tomical elements of nervous tissues, as well as of their 

 modes of development, is as yet merely in its infancy. We 

 have much to learn concerning the actual relation of 

 fibres and cells, and their different modes of continuity ; 

 our knowledge of the structural relations existing between 

 different centres in higher animals is most incomplete ; 

 and, concerning the various kinds of peripheral nerve end- 

 ings, much doubt and uncertainty also prevail. The more 

 difficult questions touching nerve evolution and development 

 are proportionately further from their ultimate solution. 



But, whatever the precise mode in which the nerve 

 cell is originally evolved in the race, or developed in the 

 embryo of any particular animal, it is perfectly certain 

 that many of these bodies are subsequently found in 

 organic continuity with nerve fibres and with one another; 

 so that (whatever other function they may fulfil) nerve 

 cells would seem to form meeting-points or termini, 

 in which different nerve currents arriving at and passing 

 through clusters of such bodies, may be brought into 

 relation with one another, and whence they are certainly 

 capable of being diverted into new directions. 



* Spencer, " Principles of Psychology," vol. i. p. 20. 



