SECT. II7.] NERVOUS SYSTEM. 137 



decussate, but pass from tlie tractus into the optic nerve of their 

 own side; and 2, commissural fibres, namely, posterior, which 

 would form a commissure of the places of origin of the nerves of 

 vision, and anterior, which can serve only to unite the retince. The 

 existence of the first-mentioned fibres is certain, although they are 

 much more scanty than the decussating elements; but that of the 

 others, also, can scarcely be denied. A commissure of the corpora 

 quadrigemina and optic thalami may, perhaps, be explained phy- 

 siologically; but even a commissure of the retince cannot be 

 regarded as impossible, since we know that the retina contains 

 grey substance and nerve-cells with branched processes, from 

 which the optic nerves arise 



The discovery of Turck (Sitz. der Wien. Akad. 185 1, March— June, 1853), that 

 in diseases of the brain or cord certain tracts of fibres degenerate, and that 

 especially compound granular cells become developed in them, appears des- 

 tined to become important for the study of the course of the fibres in the 

 central nervous system ; and I would recommend, for the investigation of 

 such cases, the employment of chromic acid preparations. 



§ 117. Membranes and Vessels of the Central Nervous System. 



A. Membranes. 1. Spinal Cord. — The Dura mater consists of 

 parallel, mostly longitudinal bundles of connective tissue, and of a 

 network of fine elastic fibres. Anteriorly, it is united with the 

 fascia longitudinalis posterior of the vertebral column ; posteriorly 

 and laterally it is free, and separated from the arches of the vertebras 

 by an interspace in which there are loose anastomosing bundles 

 of connective tissue, gelatinous fat, and cells containing serum. This 

 space also contains the well-known plexus venosi and finer vessels. 

 It is generally stated that the inner surface of the dura mater is 

 covered by an outer layer of the arachnoid ; but nothing is found 

 here except an epithelium composed of polygonal cells, and there 

 is not a trace of a special membrane. The lig amentum denticu- 

 latum possesses no epithelium, and, like the thickened stripes of 

 the pia mater, to which it is attached, has exactly the same struc- 

 ture as the dura mater. 



The arachnoid of the spinal cord consists of a single layer of 

 great delicacy and transparency, corresponding in extent to the 

 dura mater. Its outer surface is connected with the dura mater 

 by means of dense bands or more delicate fibrils; in other respects, 

 it is perfectly smooth and shining, and covered by a simple layer 

 of pavement epithelium. Its inner surface is likewise smooth, but 

 without epithelium ; it is separated from the cord and the cauda 



