454 EDWARD HORNE CRAIGIE 



It is evident that while these questions are in their present 

 unsettled condition, no final explanation can be formulated to 

 account for the facts brought out in this study. We may con- 

 sider briefly, however, the possible bearing upon them of some of 

 the rival hypotheses. 



If nervous activity is of a physical nature, involving no meta- 

 bolic changes, then all the metabolism present will be the small 

 amount required to maintain life in the protoplasm. So far as 

 appears, this should not differ markedly from the rate of meta- 

 bolism in the neuroglia, which likewise has little else to do, 

 probably, than simply to perform the ordinary vegetative func- 

 tions and passively to occupy its place. There will also be a 

 certain amount of reserve energy available for growth or repair, 

 a reserve which one may assume to be of rather more importance 

 to the neuroglia than to the nervous elements themselves, and 

 which probably will not differ very greatly in different regions. 



What, then, is the significance of the differences in vascularity 

 observed in various centers? With regard to the rather large 

 difference existing in general between gray and white matter, 

 this very condition has been brought forward as an argument 

 in support of the contention that metabolism is practically ab- 

 sent in nerve fibers, and it certainly seems to agree with it. 

 The capillaries present in the white matter will in this case be 

 concerned almost entirely with the needs of the neuroglia. The 

 richer blood supply in the tract which is largely unmyelinated 

 may be due to the presence of a greater proportion of neuroglia 

 surrounding a larger number of much thinner nerve fibers in a 

 unit volume, space not being occupied by the large bulk of 

 the myelin sheaths. The myelinated fasciculus longitudmalis 

 dorsalis, however, is much richer than is this (pyramidal) tract, 

 so that the significance of the facts is not clear. The observation 

 that about 50 per cent of the white matter is composed of myelin 

 (Donaldson and Hoke, '05; Greenman, '13, '17; Koch and Koch, 

 '17) would in itself explain the fact that the vascularity of the 

 more poorly supplied gray regions is in the neighborhood of 

 twice as great as that of the white matter of the cord. 



