2 10 Journal of Comparative Neurology. 



arachnoid space, thence via the " foramen of Magendie " to 

 the ventricles. 



Further observations on these points are therefore de- 

 sirable. 



For assistance and courtesies extended in connection with 

 these dissections and observations, acknow^ledgments are due 

 Dr. F. Kebler, pathologist, and Dr. (ieorge B. Twitchell, 

 house-physician to the Cincinnati Hospital; also to Messrs. 

 S. New^lin and J. G. Williams, students at Miami College. 



To summarize these observations I would conclude: 



1. The arachnoid membrane is a true shut sac, similar in 

 structure and function to the serous membranes of the other 

 great cavities. Its parietal layer is easily separable from the 

 dura at the vertex in the foetus and young infant, but practi- 

 cally inseparable in this region in the adult. At the base of 

 the skull it is demonstrable as a separate membrane even in 

 the adult. To assert that the parietal layer of arachnoid is 

 absent, because its subepithelial connective tissue has fused 

 at the vertex with the dura (connective tissue), is as incor- 

 rect as to describe the great omentum as one layer of perito- 

 neum, because its original four layers have become matted 

 and adherent. 



2. The arachnoid cavity communicates freely with the 

 subarachnoid space, by means of two foramina situated in 

 the visceral arachnoid, one on either side of the medulla. 

 For these I Would propose the name " lunulate foramina," 

 from their crescentic or lunulated edges, produced by the 

 attachments of fibrous bands which cross the openings trans- 

 versely. Subsequent observations, in two instances, confirm 

 the presence of the " lunulate foramina." In one of these, 

 the basilar process of the occipital and the sphenoid body 

 were cut away from the base and the dura removed, so as to 

 show the foramina in situ; thus excluding the possibility of 

 of their artificial production during the extraction of the 

 brain. 



Cincinnati, December i6, 1890. 



