470 THE TISSUES. 



The arachnoid has no connection with the lining membrane of the 

 ventricles. Finally, the free surface of the arachnoid is covered by 

 a simple scaly epithelium. The external lamina, so called, of the 

 arachnoid, is merely a precisely similar epithelium upon the dura 

 mater. 



3. The dura mater of the cord (theca vertebralis), is a whitish- 

 yellow, sometimes glistening, firm, and somewhat elastic membrane, 

 formed of parallel fasciculi of white fibrous tissue, and of a fine 

 elastic fibrous network in almost equal proportions. It is twice as 

 thick posteriorly as anteriorly ; and in the latter position is pretty 

 firmly united to the anterior common vertebral ligament, while it 

 is free on the sides and behind. Internally, the dura mater is co- 

 vered by a simple scaly epithelium alone, there being no external 

 lamella of the arachnoid. The ligamentum denticulatum has no 

 epithelium, and presents a structure precisely like that of the dura 

 mater. 



In the cranium, the dura mater is thicker and whiter than in the 

 spinal canal, and consists of two layers: 1, the external or perios- 

 *teal, and 2, the internal. The former, more laxly united to the 

 latter at an early period, is whitish-yellow and rough, and attached 

 more or less firmly to the bones, and supports the larger meningeal 

 vessels. The internal layer, or proper dura mater, is less vascular, 

 whiter, has generally a glistening tendinous aspect, and its surface 

 is quite smooth. Between the two layers, with few exceptions, the 

 sinuses are situated. The processes of the dura mater (the falx 

 cerebri and cerebelli, and the tentorium) are prolongations of the 

 internal layer. The simple scaly epithelium covering the dura 

 mater consists of cells of ^vsis to ^Viy f an inch, with rounded or 

 elongated nuclei g^ to ^V <y of an inch in diameter. 



Vessels and Nerves of the preceding Membranes. The dura mater 

 of the cord has in its substance but few vessels, though numerous 

 arteries and veins of the cord perforate it. The dura mater of the 

 encephalon is far more vascular, as already described, especially in 

 its external or periosteal layer. The sinuses in it are simple exca- 

 vations lined with an epithelium. 



The arachnoid membrane, whether of the brain or the spinal 

 cord, contains no proper vessels. 



The pia mater both of the brain and the cord, has a tolerably 

 rich capillary plexus of its own, besides supporting the very co- 

 pious ramifications of the vessels of the nervous substance. 



