THE CHARACTERISTICS OF THE INDIVIDUAL NEURONE 535 



described not only in the cell body, but extending throughout the course of 

 the processes and, in fact, from cell to cell. We are not in a position at the 

 present time fully to determine what bearing these neurofibrillae have on our 

 accepted theories of nerve function, other than that they are assumed to be 

 the conducting elements. 



The Neurone Theory. Our knowledge of the function of the ner- 

 vous system is best explained on the basis of the neurone theory, which con- 

 siders the neurone as a physiological unit. By this view each gross divi- 

 sion of the nervous system is supposed to consist of a large number of in- 

 dividual neurones,* each of which is a more or less complete morphological 





FIG. 357. Purkinje Cells from the Cerebellum of the Swallow. A, Taken in the morning; 

 B, taken in the evening. (Hodge.) 



unit capable of carrying on certain functions of its own. Each of these 

 neurones maintains physiological continuity with its associates, presumably 

 by protoplasmic contact rather than by continuity; so that well-marked paths 

 of conduction are possible throughout the extent of the particular mass of 

 which the neurone is a part, and throughout the adjacent masses. By this 

 view, paths of conduction are made up of series or chains of individual neu- 

 rones which are in physiological continuity. 



The Characteristics of the Individual Neurone. The function of 

 the nerve cell may be discussed under two headings: The function of the 

 cell body, and the function of the cell processes. 



* "According to the estimations of Meynert, the cortex of the cerebral hemisphere 

 alone contains twelve hundred millions of ganglion cells. Donaldson (The Growth of the 

 Brain, a Study of the Nervous System in Relation to Education, i2mo, London, 1897, 

 p. 159) states that for the total number of nerve cells in the central nervous system three 

 thousand millions is a moderate estimate." (Barker, the Nervous System and its Con- 

 stituent Neurones, p. 42, 1899.) 



