CENTRAL NERVOUS SYSTEM. 



207 



excited, then a cell with many dendrites should offer more receiving points 

 than one with few. It is perfectly evident, however, that in many cases the 

 dendrites are not the only pathway by which impulses may travel toward 

 the cell-body. 



Theories of the Passage of the Nerve-Impulses. — Accepting the view- 

 that, with the exceptions just noted, the nervous system is composed of dis- 

 continuous but closely approximated 

 cell-elements, it remains to explain 

 how impulses arising within the limits 

 of one element are able to influence 

 others. The relation between two 

 neurones is quite comparable to that 

 between a muscle and the nerve-fibres 

 controlling it, but the recognition of 

 that fact does not afford us much 

 assistance. 



As an hypothesis the passage of 

 the stimulus may be assumed to 

 depend on chemical changes set up 

 at the tips of the terminals and affect- 

 ing the surrounding substance, which, 

 thus affected, acts to stimulate some 

 point on the wall of the neighboring 

 cell, either along a dendrite or on the 

 cell-body itself. 



The suggestion has been made that 

 in some cases the space between two neurones may be varied by amoeboid 

 changes in the dendrites and terminals of the elements concerned. Although 

 much may be said a priori in favor of this hypothesis, good histological evi- 

 dence is still wanting. 



The structural changes which permit the stimulation of one element to 

 affect another are completed slowly, and, as we shall later see, these changes 

 continue in some parts of the human nervous system up to middle life. 



From what has just been stated, it follows that the nervous system of the 

 immature person is quite a different thing from that of one mature, since in 

 the former it is more schematic, more simple, the details of the pathways not- 

 having been as yet filled out. Moreover, considering the slow and minute 

 manner in which the central system is organized by the growth of the cell- 

 branches, it is the list place where there should be expected structural uni- 

 formity in the details of arrangement. 



B. Reflex Action. 



Conditions of Stimulation. — The conditions necessary for the generation 

 of a nerve impulse are an external stimulus acting on an irritable neurone. 

 While life exists, stimulation of varying intensity is always going on, and 



Fig. 90. — Showing at the lower edge of the 

 figure a series of basket-like terminations of axones 

 which surround the bodies of the great cells of 

 Purkinje in the cortex of the cerebellum (Ramon 

 y Cajal) : C, cell-body; N, axones; B, basket-like 

 terminations arising from cell C, and enclosing the 

 cells of Purkinje. 



