Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 133 



auris, attached externally to the tympanic membrane, and 

 inserted internally into an aperture in the periotic capsule 

 called the fenestra ovalis. This aperture leads into a cavity 

 which contains the essential part of the organ of hearing, 

 which is known as the membranous labyrinth, and lies in a cavity 

 moulded to its form in the periotic capsule. Around the mem- 

 branous labyrinth, between it and the walls of the chamber, 

 is a fluid, perilymph; within the membranous labyrinth itself 

 is a similar fluid, the endolympli. The membranous labyrinth 



FIG. 43. DIAGRAMS OF AUDITORY ORGAN. 



A. Labyrinth of frog. B. Mammalian cochlea in diagrammatic section. 

 C. Mid-chamber of mammalian ear. 



a. Summit of spire of cochlea, am. Ampulla, a. s. c. Anterior semicircular 

 canal, c. c. Cochlear canal, di. Diverticulum. eus. Eustachian canal. 

 / o. Fenestra ovalis. /. r. Fenestra rotunda. 7i. s. c. Horizontal semicircular 

 canal, in. Incus, mal. Malleus, p. s. c. Posterior semicircular canal, s. 

 Sacculus. s. t. Scala tympani. s. v. Scala vestibuli. sta. Stapes, ty. m. 

 Tympanic membrane, itt. Utriculus. 



(Fig. 43, A.) consists of: (1) a vestibule constricted into two 

 divisions, the utriculus (u.) and the sacculus (5.), on the outer 

 side of which latter is a small dilatation ; and (2) three semi- 

 circular canals, of which two are nearly vertical, and the third 



