CH. LI.] 



LABYRINTHINE IMPRESSIONS 



753 



the petrous portion of the temporal bone; and consists of three 

 parts the vestibule (1), the three semicircular canals (3, 4, 5) which 

 open into the vestibule, and the tube, coiled like a snail's shell, called 

 the cochlea (6, 7, 8). The cochlea is the part of the apparatus which 

 is concerned in the reception of auditory impressions ; it is supplied 

 by the cochlear division of the eighth or auditory nerve. The 

 remainder of the internal ear is concerned not in hearing, but in 

 the reception of the impressions we are now studying; it is sup- 

 plied by the vestibular division of the eighth nerve. Within the 

 vestibule are two chambers made of membrane, called the utricle 

 and the saccule; these com- 

 municate with one another and 

 with the canal of the cochlea. 

 Within each bony semicircular 

 canal is a membranous semi- 

 circular canal of similar shape. 

 Each canal is filled with a 

 watery fluid called endolymph, 

 and separated from the bony 

 canal by another fluid called 

 perilymph. Each canal has a 

 swelling at one end called the 

 ampulla. The membranous 

 canals open into the utricle; 

 the horizontal canal by each of 

 its ends ; the superior and pos- 

 terior vertical canals by three 

 openings, these two canals being 

 connected at their non-ampul- 

 lary ends. 



Fig. 461 shows in transverse section the way in which the 

 membranous is contained within the bony canal ; the membranous 

 canal consists of three layers, the outer of which is fibrous and 

 continuous with the periosteum that lines the bony canal ; then comes 

 the tunica propria, composed of homogeneous material, and thrown 

 into papillae except just where the attachment of the membranous to 

 the bony canal is closest ; and the innermost layer is a somewhat 

 flattened epithelium. 



At the ampulla there is a different appearance; the tunica 

 propria is raised into a hillock called the crista acoustica (see fig. 462) ; 

 the cells of the epithelium become columnar in shape, and to some 

 of them fibres of the eighth nerve pass, arborising round them ; 

 these cells are provided with stiff hairs, which project into what is 

 called the cupula, a mass of mucus-like material containing otoliths 

 or crystals of calcium carbonate. Between the hair-cells are fibre- 



3 B 



FIG. 461. Section of human semicircular canal. 

 (After Riidinger.) 1, Bone; 2, periosteum ; 3, 3, 

 fibrous bands connecting the periosteum to 4, the 

 outer fibrous coat of the membranous canal; 

 5, tunica propria ; 6, epithelium. 



