734 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



plexity almost solely by the formation of dendrons, the collaterals which come 

 from the neurons of these cells forming but an insignificant contribution. Not 

 only, therefore, is organization in large part dependent on changes in the cen- 

 tral cells by reason of their numerical preponderance, but also by reason of the 

 fact that to them a multiplication of pathways both by elaboration of the 

 neurons and the dendrons is alone possible. 



Defective Development. In view of these facts, defective development 

 in the nervous system may depend on failure in one or more of these several 

 processes by which the system is organized, and it should be possible to correlate 

 defective development involving mainly one set of elements with a distinct 

 clinical picture. The results of defective development are not merely an 

 absence of certain powers, but in some measure a diminution in the strength 

 and range of those that remain. 



Laboratory Animals. The bearing of these facts on the conception which 

 we form of the nervous systems of those animals commonly employed for 

 laboratory experiments may be here mentioned. The frog, pigeon, rabbit, 

 cat, and dog form a series in which the total mass of the central system 

 increases from the beginning to the end of the series. 



The number of cells in the largest system, that of the dog, is many times 

 greater than that in the smallest, the frog, and it is probable that the others are 

 in this respect intermediate. Organization is apparently more rapidly completed 

 and more nearly simultaneous throughout the entire system in forms like the 

 frog and pigeon, and also in these latter the organization is least elaborate at the 

 cephalic end. While the educability of the nervous system of the dog may 

 depend on several conditions, the comparative slowness of organization is 

 undoubtedly one of them, and a very important one. Where the organ- 

 ization is early established it is found that the parts organized have a 

 greater independence than under the reverse conditions. In selecting an 

 animal, therefore, on which to make a series of experiments, these several 

 facts must be kept in view, for the choice is by no means a matter of 

 indiiference. 



Blood-supply. For the general distribution of the blood-vessels in rela- 

 tion to the gross subdivision of the brain the student is referred to the works 

 on anatomy. The finest network of vessels is, however, to be found where 

 the cell-bodies are most densely congregated, and indeed the distinction 

 between the masses of gray and white matter in the central system is as 

 clearly marked by the relative closeness of the capillary network as in any 

 other way. One result of this relation between the blood-supply and the cell- 

 bodies which form the gray matter is a general arrangement of the vessels along 

 the radii of the larger subdivisions of the brain, as the cerebral hemispheres 

 and the cerebellum. 



The conditions which control the circulation within the cranium and spinal 

 canal are not exactly the same at all periods of life, but the variations occur 

 in minor points only. 



The general conditions are the following : The evidence, physiological and 



