448 SPECIAL HISTOLOGY. 



fibrosa is a whitish yellow, occasionally glistening, firm, tolerably 

 elastic membrane, consisting of parallel and mostly longitudinal 

 fasciculi of connective tissue, and of a fine, elastic, fibrous net- 

 work in almost equal proportions. The outer surface of the 

 dura mater is, in front, where the membrane is always at least 

 as thin again as behind, pretty closely united to the fascia 

 longitudinalis posterior of the spinal column, free posteriorly 

 and on the sides, and separated from the arches of the vertebrce 

 and their periosteum by a space, occupied by a lax connective 

 tissue with anastomosing fasciculi scarcely more than O004 — 

 O005'" thick (reticular connective tissue), containing a few 

 elastic fibrils (convoluted and longitudinal), and round, fusiform 

 and stellate nucleated cells, similar to the formative cells of the 

 connective tissue, and besides these with larger or smaller 

 aggregations of frequently gelatiniform, transparent fat with 

 cells containing serous fluid. The vessels of this space* are in 

 part the well-known plexus venosi, in part finer vessels, and also 

 a network of the finest capillaries in the lax connective tissue 

 itself. The internal surface of the dura mater would appear, 

 from what is generally stated, to be lined with an outer lamella 

 of the arachnoid ; nothing however is to be seen but an 

 epithelium, composed of polygonal, flattened, nucleated cells, on 

 the innermost layer of the dura mater, and not a trace of any 

 special substratum. The lig amentum denticidatum has no epi- 

 thelium, and like the thickened processes of the pia mater, to 

 which it is attached, presents, in all respects, a structure similar 

 to that of the dura mater. 



The arachnoid membrane is constituted, not of an external 

 and internal lamella, the former of which is united to the dura 

 mater, the latter free, but of a single layer corresponding to the 

 internal lamella of authors. It is an extremely delicate, 

 transparent membrane, exactly corresponding in extent and 

 relations to the dura mater. Its outer surface, in the posterior 

 mesial line of the cervical portion, is connected with the dura 

 mater, above, by tolerably strong processes, lower down, by 

 delicate fibrils, elsewhere it is perfectly smooth and glistening, 

 which appearance depends upon an epithelium precisely like 

 that of the dura mater, and it is merely in apposition with that 

 membrane, as the pulmonary pleura is with the costal. The 

 internal surface of the arachnoid is also smooth, though without 



