626 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Whether, however, the delay in the response is due to the failure of the 

 cytoplasm of the receiving cell to discharge until repeated impulses have 

 reached it, or whether the modification of the cell which causes the delay is 

 a process taking place at the point where the impulse passes over from the 

 branches of one cell to those of another, is not directly determined by the 

 experiments. The indirect evidence is, however, entirely in favor of the view 

 that the delay which is notable in the arousal of a reflex response occurs at 

 the point where the impulse passes from one cell to another. 



0. THE NUTRITION OF THE NERVE-CELL. 



The metabolic processes within the nerve-cell are continuous, and the 

 chemical changes there taking place involve not only those prerequisite to the 



enlargement of the cell during 

 growth, but also those leading 

 to the formation of such sub- 

 stances as by their breaking 

 down release the energy that 

 appears in the nerve-impulse. 

 The passage of the nerve- 

 impulses probably alters the 

 osmotic powers of the cell- 

 wall toward the surrounding 

 plasma, and this of course is 

 fundamental to the nutritive 

 exchange. It follows, there- 

 fore, that the passage of nerve- 

 impulses is one factor deter- 

 mining the nutrition of these 

 cells. 



Cell-body. Histologically 

 we look upon the cell-bodies 

 as the part in which the most 

 active changes occur, since the 

 network of blood-vessels is 

 most dense about these, indi- 

 cating that the metabolic pro- 

 cesses are here most active 1 

 (Fig. 155). 



Chemical Changes. For 

 the direct micro-chemical de- 

 termination of special sub- 

 stances within the nerve-cells there are but few methods, though some phos- 

 phorus-bearing substances (nucleins) can be demonstrated, 2 and the occurrence 



1 Shimamura: Neurologische Centralblatt, 1894, Bd. xiii. 



2 Lilienfeld und Monti: Zeitschrift fur physiologische Chemie, 1892, Bd. xvii. 



FIG. 155. Frontal sections through the human mid-brain 

 at A, level of the anterior quadrigeminum ; B, level of the 

 posterior quadrigeminum (Shimamura). On the left side the 

 blood-vessels have been injected ; on the right the gray mat- 

 ter is indicated by the heavy lines. It appears by this that 

 the blood-vessels are most abundant in the gray matter. 



