VISCERAL ANATOMY. l6l 



THE INTERNAL EAR. 



The structures of the internal ear are contained in the petrous 

 portion of the temporal bone forming the bony labyrinth, the 

 parts themselves being called the membranous labyrinth. 



The Bony Labyrinth consists of the vestibule, semicircular 

 canals and cochlea. 



The -vestibule lies inside the inner wall of the tympanum, 

 receiving the jenestra ovalis, the five openings of the semicircular 

 canals above, the commencement of the scala cochlea and the 

 openings for the vestibular branches of the eighth nerve. 



The semicircular canals are three in number, opening at five 

 points. They are named the superior, posterior, and external. 



The Cochlea (or snail shell) is situated outside and below 

 the vestibule, consisting of a central stem, the modiolus, with a 

 twisted partition, the lamina spiralis, running about it, and divid- 

 ing the cochlea into two portions, that above the partition, the 

 scala tympani, communicating with the tympanum by the jenes- 

 trum 'rotundum; that below, the scala vestibuli, entering the 

 vestibule. The lamina does not quite reach the outer wall of 

 the cochlea. 



The Membranous Labyrinth consists also of the vestibule, 

 semicircular canals and cochlea. 



The vestibule consists of two thin sacs, a superior or utricle, 

 communicating with .the semicircular canals, and inferior or 

 saccule, in contact with the former. 



The structure of the vestibule is an external fibrous coat and 

 internal epithelial lining, with some calcareous particles (the 

 otoliths) at the entrance of the nerves. 



The Membranous Semicircular Canals fill the bony canals, 

 and are of the same structure as the vestibule. 



One of their extremities is dilated, the ampulla. 



The Membranous Cochlea. 



The lamina spiralis is continued by two membranes to the per- 

 iphery of the cochlea, dividing it into the two already described 

 the s. tympani below and s. vestibuli above and a median one 

 between the two, the auditive, which contains the organ of Corti, 

 a large number of small epithelial elements, resting on the mem- 

 brana basilaris, and having above the membrane of Corti. 



