394 SPECIAL HISTOLOGY. 



fine or wanting, others again, in addition, contain elastic fibres also in 

 the interior. 



The vascular membrane , 2^'^'^ mater, very closely invests the spinal 

 cord and the gray substance of the filum terminale, penetrating on the 

 one hand into the anterior and posterior fissures, where it appears within 

 the spinal cord in the form of slender processes, and affording, on the 

 other, delicate sheaths to the roots of the nerves. It contains for the 

 most part common connective tissue with straight fibres, and, more rarely, 

 anastomosing bundles ; and besides these a good many nuclei often of a 

 lenticular form, with a few elastic fibrils. Here and there are met with 

 in the pia mater bright yellow or brown pigment-cells, of an irregular, 

 fusiform figure with fine prolonged ends and measuring 0-04-0*05 of a 

 line in length, which in the cervical region, owing to their greater num- 

 ber, give the membrane, not unfrequently, a brown or even blackish color. 

 2. Brain. — The membranes of the brain, though corresponding, in 

 general, with those of the spinal cord, yet present some differences. The 

 duj'a mater, in this situation, consisting of the true fibrous membrane of 

 that name and of the internal periosteum of the cranial bones, which, as 

 the immediate continuations of the corresponding membranes of the 

 spinal canal, become consolidated together at the level of the atlas, is, 

 in general, thicker and also whiter than in the spinal cord. Its external 

 or periosteal lamella, of a whitish-yellow color and rough, is attached 

 more or less firmly to the bones, supports 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 con- 

 nective 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 considerable distances in it, or, espe- 

 cially as in the sinuses, form small, tendinous bands, crossing each other 

 in various directions, 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 tessellated epithelial 

 cells, of 0-005-0-006 of a line in size, with rounded or elongated nuclei 

 measuring 0*002-0'004 of a line ; possessing no other covering which 

 might be described as a parietal lamella of the arachnoid {yid. Luschka, 

 Sercise Hiiute, p. 64). 



