642 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



within the central system, as is the case with those giving rise to the ventral 

 roots ; (6) those forming the peripheral ganglia entirely outside of the central 

 system the sympathetic ganglia and the more or less solitary cells which take 

 part in the formation of the peripheral plexuses. 



Relative Development of Different Parts. The bulk of the three sub- 

 divisions which have been named is by no means equal. The central system 

 is far more massive than the afferent and efferent, taken together, but the 

 relation cannot be stated with any exactness, since the mass of the peripheral 

 system is not definitely known. The afferent and efferent groups are, how- 

 ever, about equal in weight, so that the comparatively small mass of either of 

 them, taken alone, is apparent. When in addition to this disproportion it is fur- 

 ther recalled that in both the groups last named the number of cell-elements 

 is small as compared with the number which compose the central system, the 

 disproportion is still further emphasized. That the central or distributive 

 division of the nervous system is thus the most important is indicated also by 

 the fact that, in the vertebrate series, as the complexity of the entire nervous 

 system increases, the proportional development of the group of central ele- 

 ments is most marked. 



Moreover, if we take the areas of the cross sections of the various spinal 

 and cranial nerve-trunks as a measure, it is found that the areas for the afferent 

 are greater than those for the efferent elements, and that the area of afferent 

 nerves increases from the cord toward the encephalon. 



Organization. During early fetal life all the cells are isolated from each 

 other. Either they are without branches as in the earliest state, or the branches, 

 although formed, have not come into such relations with the neighboring ele- 

 ments that nerve-impulses are able to pass by way of them. The series of 

 changes by which the elements are put into the most perfect physiological 

 connection which they will ultimately attain may be designated as organization. 



This change is dependent on two structural conditions (a) the number of 

 the dendritic branches, and of the terminal and collateral branches of the 

 neurons, and (6) the relations in which these deudrons and terminal and col- 

 lateral branches stand to one another. 



In the case of cells like those of the cortex, it is to be seen from the 

 instructive figure of Cajal (see p. 612), that in the vertebrate series the cor- 

 tical cells tend to possess more branches the higher the animal stands in the 

 series, i. e. the more complicated and adaptable its reactions. Further, the 

 same figure shows that in the development of the individual cells it passes 

 from a condition in which it has few to that in which it has many branches. 

 Certainly the disposition of the cell-substance in the form of branches in- 

 creases the surface thus exposed, and, assuming that the nutrition of the cell 

 takes place over this surface generally, they increase its nutritive capacity. 

 There is, however, another and more important standpoint from which they 

 may be regarded. Cajal has suggested that the dendrons are the pathways 

 by which impulses enter the cells. If this is true, then the number of den- 

 drons characteristic of any group of cells may be taken as an index of the 



