286 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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

 an absence of certain powers, 1 >ut in some measure a diminution in the strength 

 and range of those thai remain (Hammarberg *). 



Laboratory Animals. — The 1 tearing of these facts on the conception 

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

 for laboratory experiment.- may be here mentioned. The frog, pigeon, rabbit, 

 cat, and dog form a series in which the total ma^s of the central system in- 

 creases 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. 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 organiza- 

 tion is early established it is also simple, and thus portions of the system 

 retain through life a greater capacity for acting alone. In selecting an ani- 

 mal, 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 indifference. 



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 (see p. 191). One result of this relation between the blood-sup- 

 ply 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 studies of IIuber J show that in the cat, dog, and rabbit at least, the 

 vessels in the pia of the cerebral hemispheres are supplied with both medul- 

 lated and unmedullated nerves. The former are probably sensory in func- 

 tion ; the latter, possibly, vaso-motor. These latter nerves have been fol- 

 lowed to arteries so small as to possess but two layers of muscle-cells, but 

 were not traced by Huber to vessels actually penetrating the nervous sub- 

 stance of the hemispheres, von Kolliker, however, claims to have followed 

 them even there. 



These observations make the existence of a corresponding vaso-motor 



1 Bammarberg: Studien ueber Klinik und Pathologie der f<liotie nebst Untersuchungen ueber 

 die normale Anatomie der Hirnrinde, CJpsala, 1895. 



2 Journal of Compan ill ve Neurology, 1S99, vol. ix. No. 1. 



