3o6 Journal of Comparative Neurology. 



generally accepted view up to the time of Kolliker, who 

 wrote :(^) " It is generally stated that the inner surface of the 

 dura mater is covered by an outer layer of the arachnoid; but 

 nothing is fonnd here excepting an epithelium composed of 

 polygonal cells, and there is not a trace of a special mem- 

 brane." 



The same writer (page 238), speaking of the spinal 

 membranes, says: " 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." Again, Frey(') says: 

 " The arachnoid, which has also been numbered among these 

 (the serous membranes), has no parietal layer." And, " The 

 second membrane, the arachnoidea, was formerly described 

 as forming a shut serous sac, but erroneously so; the parietal 

 leaf being usually represented as fused together with the 

 outer layer of the dura mater, since it could not be demon- 

 strated separately. "(') 



Without multiplying references unnecessarily, it is suffi- 

 cient to state further that in the various editions of Gray's 

 " Anatomy," previous to 1870, the arachnoid is described as 

 a shut sac. Darling and Ranney, 1883, also teach this view; 

 while Gray (after 1870), Holden, fifth edition, 1885; Leidy, 

 1889; Weisse, 1886, and other leading works in common use 

 as text-books, speak of it as consisting of one layer only — the 

 " visceral " layer. 



It has occurred to the writer that this question of one or 

 two layers w^as one which it was desirable to have settled, 

 and if possible by macroscopic rather than microscopic evi- 

 dence. With this object in view a series of dissections were 

 made as follows: 



First dissection: Foetus at term, still-born. — The scalp 

 being removed, a section of skull was made in the parietal 



4 " Manual of Human Microscopic Anatomy," p. 237-238. London, i860. 



5 " Histology and Histo-chemistry of Man," p. 227. Appleton, 1875. 



6 Op. cit., p. 599. 



