72 



INNERVATION. 



[CHAP. xvin. 



of the organ of hearing, and includes the ultimate distribution of the 

 nerve. It consists of three parts, the vestibule, the semi-circular 

 canals, and the cochlea, all of which, from their delicacy and mi- 

 nuteness of structure, demand careful examination. They are 

 a series of cavities hidden in the hardest part of the petrous bone, 

 communicating on the outside with the tympanum, by the fenestrre 

 ovalis and rotunda already described, and on the inside with the 

 internal auditory canal, which conveys the nerve to them. The 

 very compact bone immediately bounding these cavities, considered 

 apart from the less dense bone which surrounds it, is termed the 

 osseous labyrinth, in distinction from a membranous labyrinth 

 within. 



Of the Osseous Labyrinth. The singularly complex shape of this 

 part of the organ makes it difficult to describe. 1 . The vestibule 

 or common central cavity, placed immediately to the inner side of 

 the tympanum, is flattened from side to side, and about a fifth of 



Fig. 134. 



Osseous labyrinth of the left side. o. Fenestra ovalis, leading into the cavity of the vestibule. 

 From this a bristle, t, is passed into v v, the vestibular scala of the cochlea, which is laid open 

 in part by the removal of the outer wall. r. Fenestra rotunda, seen almost in profile. Through 

 this a bristle, J, is passed into the tympanic scala of the cochlea, t , exposed by the removal of 

 part of the membranous portion of the lamina spiralis. The three semicular canals are seen, 

 with their extremities entering the vestibule, and one end of each dilated into an ampulla. Mag- 

 nified 3J diameters. Partly from Sremmerring. 



an inch in height, as well as from before backwards. The semi- 

 circular canals open into it by five orifices behind, the cochlea by a 

 single one in front; on its outer wall is the fenestra ovalis, on its 

 inner several minute holes, including the macula cribrosa for the 



