388 



SPECIAL HISTOLOGY. 



fibres, and to the attenuation and separation of their elements, until, when 

 they have reached the gray layer, they become lost to sight, although if 

 more closely traced they may still be perceived as intricately interlaced 

 fibrils of the utmost fineness, and with scarcely any appearance of dark 

 contours, only that there are a certain, though smaller number of fibres, 

 which, upon reaching the gray layer, do not lose their breadth and 

 dark contours, but are continued in a straight or oblique course through 

 it, extending horizontally to a further distance, in the outer white 

 layer. In this layer, consequently, we find a considerable number of 

 finer, and of the very finest fibres (Fig. 150), crossing each other in 



various directions, and in 



- 15 - several superimposed layers, 



which, are obviously, as to 

 their origin, to be referred 

 to those arising from the 

 reddish-gray layer ; and 

 which probably also, as 

 Remak has assumed, are 

 derived, at the basis of the 

 cerebrum, from the anterior 

 extremity (knee) of the cor- 

 pus callosum. How these 

 fibres are related to the cells 

 in the white layer is doubt- 

 ful, although this much is 

 certain, that many of them 

 return into the gray-red 

 substance from which they 

 arose, or in other words from loops, which were first described by 

 Valentin, and which I have very frequently and distinctly noticed in 

 chromic acid preparations treated with caustic soda. I have also 

 observed, in the gray-red substance, isolated loops with closely ap- 

 proximated sides, and also with their convexity looking towards the 

 surface of the brain. The fasciculi of the gray-red substance contain 

 fibres which, at first, measure 00012-0-003 of a line, but almost all 

 of which ultimately decrease in size down to 0-001, and, in the gray 

 substance, acquire the diameter of the smallest nerve-tubes, 0'0004 

 O'OOOS of a line. The fibres given off from these fasciculi, within 

 the gray-red layer are, in part, of the same size as those in the 

 fasciculi, which is the case particularly with those of the thicker white 

 streak, in part finer. The fibres which proceed from these fasciculi 

 into the superficial white substance, are also, usually, of greater size, up 



FIG. 150. Finest nerve-tubes of the superficial white substance of the human cerebrum; 

 magnified 350 diameters. 



