450 SPECIAL HISTOLOGY. 



the larger vasa meningea, and is also otherwise more richly 

 supplied with vessels than the internal proper dura mater, with 

 which, at an earlier period, it was more laxly united, and from 

 which, except where the sinuses are contained, it may not 

 unfrequently be separated even in the adult. The internal 

 lamella is less vascular, whiter, presenting in many places a 

 glistening tendinous aspect, and on its surface is quite smooth 

 and for the most part even. The processes of the dura mater, 

 the greater and less falciform processes, and the tentorium, 

 appear as prolongations of this internal lamella; and between 

 the two lamellae are situated, with few exceptions, the venous 

 canals or sinuses of the dura mater. Both lamellae contain 

 connective tissue of the same form as that in the tendons and 

 ligaments, with, for the most part, indistinct bundles, and 

 parallel fibrils, which either extend of a uniform size for con- 

 siderable distances in it, or, especially as in the sinuses, form 

 small, tendinous bands, crossing each other in various direc- 

 tions, and containing among them a good many fine elastic 

 fibres. The internal surface of the dura mater is lined with a 

 single (according to Henle with more than one) layer of tesse- 

 lated epithelial cells, of 0-005 — 0006'" in size, with rounded or 

 elongated nuclei measuring 0'002 — - 004'"; possessing no other 

 covering which might be described as a parietal lamella of the 

 arachnoid (vid. Luschka, Serose Haute, p. 64). 



The arachnoid membrane of the brain differs from that of 

 the spinal cord, not so much in its structure as in its disposition. 

 It is true, that in this situation also, there is but one lamella 

 demonstrable as a membrane composed of connective tissue, 

 which corresponds with the so- termed visceral layer of the 

 arachnoid of authors, and is also very closely applied to the 

 inner surface of the dura mater, but the arachnoid membrane 

 here is in much more intimate relation to the pia mater. That 

 is to say, instead of its being united with the latter, as in the 

 cord, by scattered fibres and lamella?, it is, in the brain, in 

 man}'' situations, as on all the convolutions, and the projecting 

 parts at the base of the brain, adherent to and coalescent with 

 it, and, elsewhere, where this is not the case, united to it by 

 numerous processes. For this reason, there exists, in the 

 brain, no continuous subarachnoid space, but numerous, larger 

 and smaller spaces, which only partially communicate. The larger 



