VASCULAR DRAINAGE OF ENDOLYMPHATIC SAC 81 



which everywhere inclose the duct and sac. In the distended 

 state, as in injected specimens, they virtually constitute a sur- 

 rounding sheet of blood inclosed in endothelium, since the open- 

 ings in the mesh are, as a rule, narrower than the blood channels 

 themselves. There is some tendency at this time, and it becomes 

 more marked later on, to the formation of principal channels 

 in this plexus. The plexus anastomoses centrally with the other 

 blood vessels of the labyrinth. Distally it drains by several 

 openings into the transverse sinus. In addition it anastomoses 

 with a coarser plexus of veins that lies between the dura and the 

 cartilaginous skull in the neighborhood of the sinus. In this 

 same region there are some small arteries of the dura mater that 

 seem to communicate by minute branches with the endolymphatic 

 plexus. There were very few of these and their arterial nature 

 could not be determined with certainty. 



ENDOLYIVIPHATIC APPENDAGE AT END OF FOURTH MONTH 



The endolymphatic plexus gradually changes its character as 

 we advance to older fetuses. Instead of a fairly uniform mesh- 

 work that envelops evenly all parts of the appendage, part of it 

 takes the form of larger and simpler channels that become more 

 or less separated from the remainder of the plexus while the latter 

 continues as a fine meshwork closely applied to the surface of the 

 appendage. The finer plexus drains into the larger channels 

 which in turn drain into the transverse sinus. 



In order to determine the topography and vascularization of 

 the endolymphatic appendage at this period, a well hardened 

 fetus, 130 mm. crown-rump length, was selected in which the 

 blood-vessels had been injected through the umbilical vein with 

 India ink (No. 1018, Carnegie Collection). The part of the 

 skull on each side containing the labyrinth was removed, care 

 being taken to preserve the dura. The specimen from the right 

 side was dehydrated and cleared in wintergreen oil and studied 

 as a transparent specimen. The left one was decalcified and 

 cut in serial sections and a profile reconstruction was made of the 

 labyrinth and larger vessels. By combining the reconstruction 

 with the study of the transparent specimen it was possible to 



