THE EAR 



207 



and horizontal transverse planes. Both extremities of each 

 semicircular canal open into a sac, termed the utricle (Fig. 80). 



The ductus endolymphaticus issues from the saccule and 

 unites with the ductus utriculo-saccularis from the utricle. By 

 means of these connections the endolymph circulates freely 

 throughout the whole of the membranous labyrinth. 



The saccule, the utricle and their ducts are all placed in the 

 vestibule of the osseous labyrinth. 



The membranous labyrinth does not occupy the whole of 

 the available space within the osseous labyrinth, and the inter- 

 val between it and the bone is filled with a fluid, termed peri- 



DC 



FIG. 80. Diagram of the Membranous Labyrinth. (TURNER'S Anatomy.} 



DC. Ductus cochlearis. 

 S. Saccule. 



SC. Semicircular ducts. 

 U. Utricle. 



dr. Ductus reuniens. 

 dv. Ductus vestibuli. 



lymph. When the foot-piece of the stapes is moved medially, 

 waves are set up in the perilymph, and they pass up the 

 cochlea to its summit and then descend to impinge on the 

 membrane which closes the fenestra cochleae (rotunda) 

 (p. 204). As the waves pass along, they are transmitted 

 through the wall of the membranous cochlea to the endo- 

 lymph and so stimulate the processes of the cells which are 

 connected with the terminal fibres of the cochlear division of 

 the acoustic nerve. 



The terminal branches of the vestibular division of the 

 acoustic nerve end in and around specialised cells in the walls 

 of the membranous semicircular ducts. They are stimulated 

 by movements in the endolymph, and they are also affected 



