90 EQUINE ANATOMY. 



Action : Constricts the cartilaginous tube of the ear. 



Middle ear or tympanum. — Division : An external 

 wall formed by the membrana tympani, an internal with 

 the fenestra ovale separated by the promontory from 

 the fenestra rotunda. The circumference is occupied 

 by the mastoid cells. Internally are found four bones, 

 the malleus, incus, orbiculare and stapes ; they are 

 moved by the internal muscle of the malleus and the 

 muscle of the stapes ; this cavity is lined by a tine 

 mucous membrane and communicates with the pharynx 

 by the Eustachian tube or guttural ducts of the tym- 

 panum, a long canal cleft inferiorly, through which 

 the mucous membrane escapes and descends to form the 

 guttural pouches. 



Internal ear or labyrinth. — Division : Osseous 

 labyrinth ; 1 — a vestibule with eight orifices, the fen- 

 estra ovale, five openings of the semi-circular canals, 

 the opening of the seala cochlea and the forami- 

 na for the branches of the acoustic nerve ; 2 — the 

 semi-circular canals, divided into superior, posterior and 

 external. 3 — The cochlea or snail shell twisted round 

 the modiolus and divided by the lamina spiralis into 

 superior and inferior scalse. 



Membranous labyrinth. — 1. Membranous vetibule : 

 formed by two sacs, the superior or utriculus, and the 

 inferior or sacculus ; and composed of two layers, the 



