318 (JEOIKiE L. STREETER 



that iiu'csts tlio central hcinous f^ystein. 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,^ correspond exactly with the appearances seen in the 

 histogenesis of the perioticular spaces in the ear. The perioticu- 

 lar spaces are not however extensions of the arachnoid spaces 

 that have invaded the cavity of the cartilagenous labyrinth. If 

 this were so we should find them first appearing among the root- 

 lets of the vestibular and cochlear nerves along which the sub- 

 arachnoid space extends for some little distance. Instead, they 

 begin at points where there can be no connection with the arach- 

 noid tissue and their direction of growth is quite independent of 

 it. The perioticular 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 communication becomes established between the scala tympani 

 and the subarachnoid space near the fenestra cochleae, the so- 

 called aquaeductus cochleae. Vague and conflicting statements 

 are also made concerning a communication through the internal 

 auditory meatus connecting the arachnoid spaces with the sca- 

 lae. Such communications must be estabhshed quite late. In 

 the oldest fetus examined, 130 mm. CR length, they do not yet 

 exist. As to the latter communication it can be seen that the 

 arachnoid spaces extend peripherally through the internal audi- 

 tory 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 scalae, and there is no evidence at 

 this stage that there is ever to be a communication between them 

 and the scalae. As to the aquaeductus cochleae in the 130 mm. 

 fetus it can be plainly seen that it is already forming as a deriva- 

 tive of the arachnoid spaces although the communication with 

 the scala tympani is not yet established. The arachnoid spaces 



' Weed, L. H. The development of the cerebro-spinal spaces in pig and in 

 man. Contributions to Embryology, vol. 5, No. 14, Publications of Carnegie 

 Inst, of Wash., No. 225, 1917. 



