6 5 2 



THE SENSES 



traction of the tensor tympani muscles. The stapes is movable on the proc- 

 ess of the incus, the contractions of the stapedius muscle draws it outward. 

 The axis round which the malleus and incus rotate is the line joining the 

 processus gracilis of the malleus and the posterior process of the incus. 



The Internal Ear. The internal ear, or labyrinth, constitutes the 

 proper organ of hearing. It contains special epithelial structures to which 

 are distributed the auditory nerves. The organ is located in a cavity in the 



FIG. 435. Membranous Labyrinth of a 30 mm. Human Fetus. A , Viewed from its Lateral 

 Aspect; B, viewed from the mesial aspect. (Streeter.) 



petrous bone, called the osseus labyrinth. The auditory organ within is 

 called the membranous labyrinth. The membranous labyrinth contains a 

 fluid called endolymph; while outside it, between it and the osseous labyrinth, 

 is a fluid called perilymph. This is not a pure lymph, as it contains mucin. 

 The osseous labyrinth consists of three principal parts, namely the vesti- 

 bule, the cochlea, and the semicircular canals, containing the respective 

 divisions of the membranous labyrinth. The osseous labyrinth possesses 

 openings on its inner wall for the entrance of the divisions of the auditory 

 nerve from the cranial cavity, in its outer w r all the fenestra ovalis, 2, figure 

 433, an opening filled by the base of the stapes, and the fenestra rotunda. 

 The vestibule also presents an opening, the orifice of the aqueductus vestibuli. 



