THE NERVOUS SYSTEM. ll-'J 



epithelium ; it is separated from the spinal cord, and cauda 

 equina by a large interspace, the subarachnoid space, affording, 



however, numerous slender processes to the pia mater and the 

 roots of the nerves, which processes not only accompany the 

 vessels and nerves, but occur, especially in the posterior mesial 

 line, arranged in a consecutive series, and occasionally, parti- 

 cularly in the cervical region, form a perforated or complete 

 septum. As regards its intimate structure, the arachnoid 

 contains, chiefly, reticularly anastomosing bundles of connective 

 tissue of 0"001 — O'OCM"', which are so united as to form 

 lamellae, some external with more slender, and some internal 

 with stronger fasciculi, and which are usually so surrounded by 

 fine elastic fibres, as to present a moniliform appearance when 

 swollen by the application of acetic acid (fig. 23). In many 

 fasciculi, these fibres are very fine or wanting, others again, 

 in addition, contain elastic fibres also in the interior. 



The vascular membrane, pia mater, very closely invests the 

 spinal cord and the grey substance of the filum terminate, 

 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 O04 — O05"' in leugth, which 

 in the cervical region, owing to their greater number, give the 

 membrane, not unfrequently, a brown or even blackish colour. 



2. Brain. — The membranes of the brain, though corre- 

 sponding, in general, with those of the spinal cord, yet present 

 some differences. The dura 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 con- 

 tinuations 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 colour and 

 rough, is attached more or less firmly to the bones, supports 



i. 29 



