486 



HISTORY OF THE HUMAN BODY 



ectoderm, they gradually fold in and form a deep pit, which, 

 as the process continues, pushes further into the interior, where 

 it expands into an otic vesicle, retaining its connection with the 

 exterior, however, through a narrow tube, the ductus endolym- 

 phaticus. In selachians this connection is retained throughout 

 life, and a minute but evident external pore is found near the 

 top of the head which communicates through a small duct with 

 the interior of the labyrinth ; but in all other cases the connec- 

 tion with the exterior becomes severed and the endolymphatic 



FIG. 132. Development of human otic capsule. Drawn from models 

 by F. ZIEGLER, after WM. His. 



duct ends in a somewhat expanded blind sac, the saccus endo- 

 lymphaticus. In mammals the endolymphatic duct is lodged 

 in a canal of the petrosal bone, the aqueductus vestibuli, and 

 enters the cranial cavity, where its terminal sac lies just be- 

 neath (i.e., outside of) the dura mater.* 



*In some cases the ductus endolymphaticus and its terminal sac attain 

 a high degree of development and come into association with organs re- 

 mote from its place of origin. Thus in certain teleosts the two ducts 

 unite into a median sinus which is connected with the air bladder by a 

 chain of four ossicles (Weber's apparatus), developed from the ribs of 

 the first four vertebrae. By this means the degree of fullness of the air- 

 bladder may be perceived. 



In the Anura the endolymphatic ducts form a common sinus, which 



