578 



A MANUAL OF VETERINARY PHYSIOLOGY 



Fig. 181. — Diagrammatic Section of the 

 Horse's Ear. 



i, External auditory canal ; 2, the tym- 

 panum ; 3, chain of bones across the 

 middle ear ; 4, the Eustachian tube ; 5, the 

 internal ear ; the number is on the vesti- 

 bule, above which may be seen the semi- 

 circular canals, while below is the cochlea. 



contained in a solid piece of bone in which two small foramina or 



windows exist, one known as the fenestra ovalis, the other the 



fenestra rotunda ; the base of the stapes or third bone of the ear is 



attached to the membrane 

 which covers the fenestra 

 ovalis. 



All three parts of the laby- 

 rinth communicate, but it is 

 quite certain that all three do 

 not take an equally active 

 part in hearing. The cochlea 

 alone is the essential organ 

 of hearing. The whole of the 

 labyrinth is lined by a mem- 

 brane containing a fluid 

 known as the peri-lymph; this 

 peri-lymph has free access to 

 all parts of the inner ear. 

 Within this membrane is a 

 membranous labyrinth, the 

 counterpart of the semicircu- 

 lar canals and vestibule, and 

 this also contains fluid known 

 as endo-lymph. 



Two windows exist in the 

 bony labyrinth. The base of 

 the stapes lies over one of 

 them, and between the stapes 

 and the peri-lymph is the 



membrane which lines the internal ear. Every movement of the 



tympanum causes the bony bridge to oscillate, and every oscillation 



of this thrusts the stapes against the 



membranous window, and so sets up 



oscillations in the peri-lymph which are 



transmitted throughout the internal ear. 

 The cochlea resembles in appearance 



the shell of a snail, its interior being 



divided into three spiral channels, which 



wind their way from base to apex like 



a circular staircase. The number of 



twists in the cochlea is two and a half ; 



the axis around which these wind is 



composed of soft bone, having canals 



up which the auditory nerve travels. 



If a spiral of the cochlea be cut across The semicircular canals are to 



(Fig. 183) the three canals it contains the right, the cochlea to the 



are seen. These are divided by septa ; 



one septum, known as the lamina spiralis, 



separates the upper canal or scala vesti- 



buli, from the lower one or scala tympani. 



The third, or middle canal, is of a 



triangular shape and called the cochlear 



canal ; it contains the essential organs 



of hearing, and lies between and to the outside of the other two. 

 The roof of the cochlear canal is formed by a piece of tissue known 

 as the membrane of Reissner, while its floor, on which is situated 



Fig. 182. — The Labyrinth] 

 (Edmunds). 



left ; both windows may be 

 seen, the fenestra rotunda be- 

 ing the lowermost. The groove 

 across the body of the organ 

 lodges the auditory nerve. 

 The figure is enlarged. 



