8 NERVOUS SYSTEM OF VERTEBRATES. 



Closely related to the embryological methods already men- 

 tioned is the study of the processes at work in the degeneration 

 and regeneration of cut nerve fibers. This may be prosecuted 

 on either embryos or adults and throws especial light upon the 

 character of neurones, their histogenesis, their relations to accessor}- 

 structures and the mode and meaning of histological differenti- 

 ation within the nervous system. 



Recent work upon the rise and histogenesis of nervous elements 

 and organs has shown that the embryological methods may be 

 extended by experimental procedures to a degree little dreamed 

 of a few years ago. Many problems of development and differ- 

 entiation in the nervous system are capable of re-examination 

 by experimental embryological methods, which will especially 

 throw light on the causes at work in these processes. 



Physiological methods. — ^These may be either experimental 

 or observational. In the one case nervous actions are studied 

 under conditions determined by operative or other artificial means. 

 In the other case actions are studied under natural conditions 

 believed to be sufficiently known. Among the simpler forms 

 of experiments are those to determine the course of impulses. 

 By the help of operations, or otherwise, suitable stimuli are applied 

 to definite sensor}^ areas, to nerve trunks or roots, to certain 

 nerve centers, etc., and the actions called forth are observed and 

 studied. The method is widely applicable and has been used 

 independently for the investigation of the pathways of impulses 

 and hence of functional connections in the nervous system. For 

 its intelligent use it presupposes thorough anatomical knowledge 

 and the most reliable results are obtained by combining one or 

 more anatomical methods with it. Conduction paths may be 

 determined by this method independently and some indications 

 obtained as to the number and arrangement of their several steps. 

 These data may then be tested and confirmed anatomically. 



The study of the rate, direction and mode of conduction of 

 impulses in neurones and neurone-chains has been carried on by 

 stimulation under normal and experimental conditions. Oppor- 

 tunity is offered for the extension of this field in the direction of 

 experimentation with chemical conditions combined with cyto- 



