730 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tern. 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 

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



FIG. 206. To show in the developing human cortex the increase in the number and size of the 

 mature cell-bodies, as well as the separation of them from one another (Vignal) : A, fetus of twenty- 

 eight weeks ; B, fetus of thirty-two weeks ; C, child at birth ; D, man at maturity ; I-V, layers of the 

 cortex according to the enumeration of Meynert. 



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 medullated, 

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

 increase in its bulk. In the mature fibre the axis-cylinder and the medullary 

 sheath have nearly equal volumes, and therefore approximately equal weights. 

 The medullated fibres form probably not less than 90 per cent, of the total 



