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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Theories of the Passage of the Nerve-impulse. Accepting the view 

 that the nervous system is composed of discontinuous but closely approxi- 

 mated cell-elements, it remains to explain how impulses arising within the 

 limits of one element are able to influence others. 



As an hypothesis, this may be assumed as dependent on chemical changes 

 set up at the tips of the terminals and affecting the surrounding substance, 

 which, thus affected, acts to stimulate the neighboring dendrons. As this is 

 only an hypothesis, it may be left with the statement that it seems to fit in 

 large measure the group of facts which it is necessary to explain. 



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 last place where there should be expected structural uni- 

 formity in the details of arrangement. 



B. THE PHYSIOLOGICAL ANATOMY OP THE NERVOUS SYSTEM. 



It follows from what has already been stated concerning the relations of 

 cell-elements, that the impulse which enters the central system along a given 

 dorsal neuron is bound to be first delivered to those cells in the neighborhood 

 of which the branches of the neuron terminate. 



Therefore, in determining the course that the impulses take, the determina- 

 tion of the mode in which the dorsal root-fibres are distributed is the first step. 



FIG. 165. Schema of the human spinal cord : D. E, dorsal root, right side ; Col, collaterals from the 

 dorsal root-fibres; D. C, dorsal columns; P, crossed pyramid ; .F, direct pyramid; C, direct cerebellar 

 tract; A, antero-lateral tract. 



Afferent Roots. The manner of this termination is shown in Figures 151 

 and 165. 



Here the afferent neuron having entered into the cord is seen to divide, and 





