VASCULAEITY IN THE NERVOUS SYSTEM 457 



while the cells of the spinal cord survive for three-quarters of 

 an hour or more (Cannon and Burkett, '13). 



If it be assumed that the observed differences in vascularity 

 are indicative of corresponding differences in rate of metabolism, 

 the latter may be accounted for in either of two ways. Provided 

 that the rate of metabolism is markedly greater in the nervous 

 elements than in the neuroglia (for which there seems to be no 

 particular reason if nervous processes are purely physical in 

 nature), this difference may mean simply that the nerve cells 

 are packed together more closely in one case than in another — 

 that there is a greater volume of nervous protoplasm present 

 in a unit volume of the tissue. This is practically the view 

 stated by Obersteiner (loc. cit.), who says: "the richer any region 

 is in nerve cells the closer is the capillary network which supplies 

 it." There can be no doubt that this factor plays a part, some 

 of the centers having a considerable admixture of white matter, 

 and the cells certainly being more numerous in some parts than 

 in others. The facts, however, do not seem to be adequately 

 met by this view, and we are impelled to look about for something 

 else. 



The only other explanation which offers itself appears to be 

 that the difference in rate of metabolism deduced from the differ- 

 ence in vascularity corresponds to a difference in functional 

 activity. This implies the acceptance of the view that nervous 

 processes involve metabolic changes, for which belief, as we have 

 seen, the physiological evidence is as yet inconclusive. Even 

 if it be true, one may well wonder why the activity of sensory 

 nuclei should be greater than that of motor centers and why 

 there should be such marked differences within each of these 

 groups. Evidently, our knowledge of nervous processes will 

 have to advance a long way before these matters can be fully 

 understood. It may, however, be suggested, with all due diffi- 

 dence, that sensory centers are in more or less constant receipt 

 of stimulation, while the activity of motor centers is rather 

 intermittent. On the other hand, it may be pointed out that 

 the motor centers which are concerned with muscle tone must 

 be in a state of more or less continuous excitation. 



