THE NERVOUS SYSTEM. 393 



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 fas- 

 cia longitudinalis posterior of the spinal column, free posteriorly and on 

 the sides, and separated from the arches of the vertebrae and their peri- 

 osteum by a space, occupied by a lax connective tissue with anastomosing 

 fasciculi scarcely more than 0-004-0-005 of a line thick (reticular con- 

 nective tissue), containing a few elastic fibrils (convoluted and longitu- 

 dinal), 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 sur- 

 face 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 ligamentum denticulatum has no epithelium, 

 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 inter- 

 nal 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 correspond- 

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

 cauda equina by a large interspace, the subaracJmoid 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 consecu- 

 tive series, and occasionally, particularly in the cervical region, form a 

 perforated or complete septum. As regards its intimate structure, the 

 arachnoid contains, chiefly, reticularly anastomosing bundles of con- 

 nective tissue of 0-001-0-004 of a line, 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 appli- 

 cation of acetic acid (Fig. 23). In many fasciculi, these fibres are very 



