26 ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



One looks upon the ganglion-cells and their branches as elements which 

 bear the function of the central nervous system. Even in very low-ranked 

 animals they appear isolated or gathered into knots: ganglion-knots. Ac- 

 cording as these ganglia lie isolated peripherally or collected in a particular 

 arrangement and Joined with each other by nerve-trunks they are classified 

 as peripheral ganglia or as central nervous system. In general, it is recog- 

 nized that in the animal kingdom there is a tendency toward the gathering 

 of many ganglia into a single nervous system. The higher the rank, the 

 larger is this system; but until the vertebrate rank is reached there are im- 

 portant parts of the nervous system always more or less separated and 

 functionally as well as anatomically more or less independent. 



Physiology shows how not only the individual ganglia which lie in the 

 intestines function with relative independence, but how even structures like 

 the spinal ganglia, frequently reckoned in with the central system, still 

 enjoy relative independence from it functionally. 



What we know of the anatomical structure and of the functions of the 

 central nervous system of vertebrates forces us more and more to the conclu- 

 sions (1) that even individual parts of the central system are themselves in 

 a position to function to a certain extent independently, and (2) that even 

 the brain and spinal cord of vertebrates are composed of a series of centers. 

 Whether the one or the other of these is more highly developed, whether 

 they are in connection with deeper centers, and whether they have con- 

 nections among themselves and with higher centers determine the measure 

 of the higher or lower development of the central system. We will find 

 later that, in the course of the development of a class, individual centers 

 connected with the central nervous system have reached a high development, 

 while others have arrived at a certain stage (or reached a certain type) 

 where they remain stationary and throughout all subsequent posterity re- 

 main everywhere alike. 



One can conceive that in its essentials every nervous system is com- 

 posed of afferent tracts and efferent tracts and of tracts which form the 

 connections of the elements among themselves. 



A good insight into the complete structure of a single ganglion may be 

 gained by a study of the opposite figure (Fig. 8). It represents the entire 

 first abdominal ganglion of the ventral nerve-cord of a river cray-fish, and 

 owing to the comparative simplicity of the relations permits an insight into 

 the details. We have here a sort of schema of a central nervous system, and 

 gain at once an outlook over an entire mechanism adapted for the exercise 

 of the functions of a central organ. 



The nervous system of a cray-fish, like that of all arthropoda, consists 

 of a great niimber of separate ganglia, which are united together by longer 

 or shorter commissures. From the various-sized nerve-cells {d, e, f) there 



