160 EQUINE ANATOMY. 



The Mastoid Cells are small cavities communicating with the 

 tympanum and lined by a reduplication of its mucous membrane. 



The Ossicles of the tympanum are four in number, the malleus, 

 incus, os orbicular e and stapes. 



The malleus (hammer) shows a head for articulation for the 

 incus, a handle for articulation with the membrane, and a neck 

 showing two processes of insertion (processes gracilis and brevis). 



The incus (anvil) shows a body and two processes, the superior 

 ending in a blunt point, the inferior being attached to the orbicular 

 bone. 



The os oroiculare, a small, disk-shaped bone, lies between the 

 incus and stapes. 



The stapes (stirrup) is shaped like a stirrup and has a head 

 articulating with the preceding, two branches, united at a base, 

 which fills in the fenestra ovalis. They are held together by 

 unimportant ligaments. 



Muscles. (Two of importance only.) 



Tensor tympani. 



Origin. From Eustachian tube. 



Insertion. Into neck of malleus. 



Action. Carries handle of malleus inward, thus making mem- 

 brana tympani tense. 



Stapedius. 



Origin. Wall of tympanum, near f. vestibuli. 



Insertion. Head of stapes. 



Action. Tensor of membrana. 



The Eustachian tube is a canal of cartilaginous material running 

 from the tympanum downward for a distance of jour inches, to 

 near the guttural openings of nasal cavity, where it is expanded 

 into the guttural pouches. It is covered by the stylo-pharyngeus. 



Function. Maintains the equilibrium of air inside the mem- 

 brana tympani. 



The guttural pouches are two elongated cavities communicating 

 with the pharynx, and lying on its sides. 



They are in relation externally to the carotid and parotid glands, 

 and have a capacity of three-fourths of a pint, though capable of 

 much greater distention. 



The mucous membrane is thick, strong and well lubricated with 

 mucus. 



Function. Probably connected with hearing in some way, and 

 also with phonation. 



