CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 47 



COMMUNICATION OF PERIOTIC SPACES WITH ARACHNOID SPACES. 



The relation of the scala tympani and scala vestibuli to the subarachnoid 

 spaces surrounding the hind-brain is of considerable interest, both on account of 

 the possibility of their functional relationship and on account of the similarity that 

 exists in their development. For a satisfactory investigation of the establishment 

 and the character of the communications that are formed between these two allied 

 systems of tissue-spaces, one should resort to other methods than those used in 

 the present study, and, furthermore, one should examine older fetuses than those 

 described here. In fact, a problem lies here that would be well worth careful study. 



Certain observations, however, were made in the course of the above investi- 

 gation that bear relation to these matters, and they will be briefly outlined here. 

 In the first place, the histological picture of the periotic reticulum is essentially the 

 same as that of the early stages of the pia-arachnoidal tissue investing the central 

 nervous system. The enlargement of the meshes of the latter and the formation 

 of the subarachnoid spaces and the arachnoid cistern, as has been recently described 

 by Weed (1917), correspond exactly with the appearance seen in the histogenesis 

 of the periotic spaces in the ear. The periotic spaces are not, however, extensions 

 of the arachnoid spaces that have invaded the cavity of the cartilaginous labyrinth. 

 If this were so we should find them first appearing among the rootlets of the vestib- 

 ular and cochlear nerves, along which the subarachnoid space extends for some 

 little distance. Instead, they begin at points where there can be no connection 

 with the arachnoid tissue and their direction of growth is quite independent of it. 

 The periotic spaces may be analogous to the arachnoid spaces, but they are not 

 identical with them, nor are they an extension of them. 



According to the descriptions of the adult anatomy of the ear, a communica- 

 tion becomes established between the scala tympani and the subarachnoid space 

 near the fenestra cochleae, the so-called aqua?ductus cochlea?. Vague and con- 

 flicting statements are also made concerning a communication through the internal 

 auditory meatus connecting the arachnoid spaces with the scalse. Such communi- 

 cation must be established quite late. In the oldest fetus examined, 130 mm. 

 crown-rump length, they did not yet exist. As to the latter communication, it 

 can be seen that the arachnoid spaces extend peripherally through the internal 

 auditory meatus along the trunk of the acoustic nerve-complex, and slender pockets 

 and clefts from them extend along the larger bundles of the cochlear nerve; they 

 terminate, however, before reaching the margins of the scalse, and there is no evi- 

 dence at this stage that there is ever to be a communication between them and the 

 scalse. As to the aquaeductus cochlea?, in the 130 mm. fetus it can be plainly seen 

 that it is already forming as a derivative of the arachnoid spaces, although the com- 

 munication with the scala tympani is not yet established. The arachnoid spaces 

 invest the glossopharyngeal nerve and extend down along its trunk and pass directly 

 by the region of the fenestra cochlea? (rotunda). A thin-walled tubular pouch 

 projects from these spaces, leaving the nerve trunk and extending obliquely toward 

 the scala tympani in a direction that would meet it just distal to the fenestral 



