618 



THE SENSES 



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 wall the jenestra 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, 



The Membranous Labyrinth. The membranous labyrinth cor- 



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

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



responds generally with the form of the osseous labyrinth, so far as regards 

 the vestibule and semicircular canals, but is separated from the walls of these 

 parts by perilymph, except where the nerves enter into connection within it. 

 The labyrinth is a closed membrane containing endolymph. 



The Utriculus and the Sacculus. The vestibular portion of the inner ear 

 consists of membranous sacs, the upper, the utriculus, the lower called the 

 sacculus. The former is connected with the semicircular canals, the latter 

 with the cochlea by the cochlear canal. The utriculus and the sacculus have 

 on their floors a special patch of sensory epithelium called the macula. The 

 fibers of the vestibular divisions of the auditory nerve end in the maculae, 



