„ -jtf NERVOUS SYSTEM. [SECT. 1 1 7. 



equina by a large interspace, the sub-arachnoid space, but gives off 

 numerous bands to the pia mater and the roots of the nerves, 

 which, besides accompanying vessels and nerves, are especially well 

 marked in the posterior median line, where they are collected in a 

 linear series, so as in some measure to form a fenestrated, or even 

 a perfect partition, especially in the neck. With reference to its 

 intimate structure, the arachnoid principally contains reticulated, 

 anastomosing bundles of connective tissue, of crooi"' to o - oo4"' in 

 diameter, and scanty, fine elastic fibres. 



The pia mater closely embraces the spinal cord and the grey 

 substance of the filum terminate, passing, on the one hand, into the 

 spinal cord at the anterior and posterior fissures, in the form of 

 thin processes, and on the other, furnishing delicate sheaths to 

 the roots of the nerves. It contains for the most part ordinary 

 connective tissue, along with a few elastic fibres. In the pia mater 

 of the cervical portion of the spinal cord, and also in other parts, 

 fusiform or stellate brownish pigment cells are found, which, in 

 the former place, frequently impart to it a brown colour. 



2. Brain. The investments of the brain agree generally with 

 those of the cord, but present certain differences. The dura mater 

 consists here of the proper dura mater and the periosteum of the 

 inner surface of the cranial bones, which are immediate continua- 

 tions of the corresponding membranes of the vertebral canal, and 

 coalesce at the level of the atlas. The cranial dura mater is 

 in general thicker and whiter than that of the cord. Its outer, or 

 periosteal lamella, which is whitish yellow and rough, is attached 

 more or less firmly to the bones, supports the larger vasa meningea, 

 and is richer in vessels than the inner proper dura mater, with 

 which, at an earlier period, it is only loosely connected, and from 

 which, except in the situation of the sinuses, it can be partially 

 separated, even in the adult. The inner lamella has fewer vessels, 

 is whiter, in many places with a tendinous lustre, perfectly 

 smooth, and for the most part even, upon its inner surface. The 

 processes of the dura mater, — the large and small falx, and the 

 tentorium cerebelli — appear as prolongations of this inner lamella, 

 and between the two lamelhe the sinuses of the dura mater are 

 situated with but few exceptions. Both lamella contain connective 

 tissue of the same form as that in the tendons and fascia?. The 

 inner surface of the dura mater is covered with a multiple layer 

 of pavement epithelium cells, but has no other investment which 

 could be regarded as a parietal lamina of the arachnoid. 



The arachnoid membrane of the brain, likewise, presents only a 



