5» GEORGE L. STREETER 



or the dorso-median wall as the base of the skull becomes more 

 flattened out. The sac does not become incorporated with the 

 rest of the labyrinth in the cartilaginous capsule, but like the 

 sinus lies exposed in the floor of the posterior cerebral fossa and 

 is covered in only by the dura mater. 



Throughout the greater part of foetal life the endolymphatic 

 appendage is ensheathed by a vascular plexus, the plexus endo- 

 lymphaticus, which anastomoses on the one hand with the vessels 

 of the rest of the labyrinth and on the other hand with the 

 transverse sinus into whiqh it drains through several openings. 



This plexus makes its appearance at about the time of the dif- 

 ferentiation of the appendage into its adult subdivisions of duct 

 and sac. It can be plainly recognized in embr3^os 30 mm. long. 

 In embryos 50 mm. long, it is well developed and at that time 

 it forms a closely meshed web completely investing the append- 

 age, whereby the latter is virtually inclosed in a sheet of blood 

 from which it is separated only by the endothelium of the blood 

 spaces. 



In the course of its further enlargement and development in 

 embryos 100 mm. long and over, the endolymphatic plexus be- 

 comes resolved into a few principal channels connected with 

 which there remain parts of the original plexus. The plexus 

 persists notably in the neighborhood of the endolymphatic sac. 



One of the most constant channels that are developed through 

 the endolymphatic plexus is the one forming the so-called vena 

 aquaeductus vestibuli. This forms along the side of the endo- 

 lymphatic duct and the posterior margin of the endolymphatic 

 sac, and it constitutes a direct communication between the vas- 

 cular plexus surrounding the labyrinth on the one hand, and the 

 transverse sinus on the other. It may be a single or multiple 

 channel. Through it is drained the plexus of the endolymphatic 

 sac and also some of the dural veins of the immediate neigh- 

 borhood. 



