THE NERVOUS SYSTEM. 



239 



The internal ear, or labyrinth, consists of three parts- 

 the vestibule, cochlea, and semicircular canals (Figs. 

 1 8 and 107). All of these parts are of membrane and 

 lie in cavities of corresponding shape within the petrous 

 bone. A lymphoid fluid, the perilymph, floats the deli- 

 cate membranous internal ear within its bony cavity, 

 while within the membrane is a similar fluid, the endo- 

 lymph. The 'vestibule is a small sac adjacent to the 

 tympanum, and may be seen by looking through the 



FIG. 108. SECTION OF THE COCHLEA OF THE CALF. 10. (From 



Ellenberger, after Kolliker.} 



a, Modiolus; c, scala tympani; v, scala vestibuli; Im, lamina spiralis; pt, 

 portion of the petrous bone; r, scala media, or ductus cochlearis. 



fenestra ovalis. From the dorso-caudal aspect of the 

 vestibule, arch three semicircular canals at nearly right 

 angles to one another. The external semicircular canal 

 is in a horizontal plane and surrounds a small fossa 

 almost caudad of the fenestra ovalis. The superior 

 semicircular canal lies in a transverse plane caudad 

 to the preceding. The posterior semicircular canal 

 lies in a vertical longitudinal plane, immediately laterad 

 from the jugular foramen. 



The cochlea is a coiled canal lying within the coiled 



