CENTRAL NERVOUS SYSTEM. 283 



thirty-third year than at earlier periods, but in old age this number is 

 decreased. At exactly what age decrease sets in, is not to be determined 

 from these observations. They show, simply, that in general the number of 

 fibres was less at seventy-nine years than at thirty-three years. 



In a similar way Kaes has shown 1 that the association fibres of the human 

 cerebral cortex form three parallel systems. In general it is the deepest 

 layer — i.e., that farthest from the surface of each system — which first becomes 

 medullated. The first fibres appear at about the fourth month of life in the 

 deepest portion of the deepest layer. The middle system is the last to be 

 completely medullated, this process continuing in it up to the forty-fifth 

 year of life. 



Passow 2 has shown that at maturity the cortex of the central gyri exhibits 

 association fibres which increase in abundance as we pass from the great 

 longitudinal fissure (leg area) toward the Sylvian fissure, these fibres being 

 most abundant in the areas for the hand and fingers. On the other hand, in 

 the central gyri of a child fifteen months of age, these fibres are equally 

 abundant in these two localities. From this it appears that the differentia- 

 tion takes place after the first year of life. 



Significance of Medullation. — Two sorts of nerve-fibres are described — 

 those with and those without a medullary sheath. Both have the power of 

 isolated conduction, but in the peripheral system the non-medullated fibres 

 are found in connection with the sympathetic system, where less specialized 

 functions are carried on, and also in a large but varying degree in the central 

 system. The wider significance of this difference in medullation is at the 

 moment quite obscure. 



The first suggestion, that absence of the medullary sheath is an immature con- 

 dition which persists in various parts of the nervous system, brings us at once 

 to the question of the physiological difference thus implied, but not explained. 



It is known that the central system is at birth very imperfectly medul- 

 lated, and the growth of these medullary sheaths must form a large part of 

 the total increase in its bulk. In the mature nerve-fibre the axis-cylinder 

 and the medullary sheath have nearly equal volumes, and therefore approxi- 

 mately equal weights. The medullated fibres form probably not less than 

 97 per cent. 3 of the total weight of the nerve-tissues composing the encepha- 

 lon, and of this nearly one-half would be medullary substance. 



Increase in the Mass of the Neurones. — The amount of this increase 

 under various conditions has already been discussed, and been found to range 

 between zero and fifty-thousand-fold (p. 17(i>. 



Number of Cells. — Any attempt to determine the entire number of 

 nerve-cells, the bodies of which lie within the wall.- of the neural tube, must 

 be open to many sources of error. 



L Kaes: Wiener med. Woehenschrifl, 1895, NTos. 41,42; Kaes: Jahrbiichern der Hamburg, 

 Stoats JSsankenanstallen, J ahrgang, 1893-94, Bd. iv. 

 2 Passow: Archivfiir Psychiatrie, Bd. 31, 8. 859,860. 

 'Thompson: Journal of Comparative Anatomy, W'.i, vol, i\.: Donaldson: Ibid., No. '2. 



