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PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



labyrinth. It connects interiorly with the fauces by the Eustachian tube, 

 and posteriorly with the mastoid cells of the mastoid portion of the tem- 

 poral bone. In some animals these mastoid cells are greatly developed 

 and so form an important augmentation of the tympanic cavity, while the 

 Eustachian tube, which is short and straight in the case of most rumi- 

 nants, is very much dilated in the horse where it forms what may be 

 termed guttural pouches (Fig. 401). 



Fig. 402.— I. The Mechanics of the Auditory Ossicles, after Helm- 

 holtz. II. Section of the Middle Ear, after Hensen. (Munk.) 



I. a, malleus ; h . incus : am, long process of incus ; n, stapes ; the arrows show the direction of mo- 

 tion. II. G, external auditory canal; M.I., membrana tympani; C, tympanum; //, malleus; L.S.. 

 superior ligamant ; S, stapes. 



Stretching across the middle ear from one side to the other is the 

 chain of bones, each named from its resemblance to some instrument; 

 thus, the malleus, so-called from its resemblance to a hammer, is attached 

 to the membranum of the tympanum by its handle (Fig. 402). The second 



bone, from its resemblance to an anvil, is 

 called the incus, and is attached on the 

 one side to the malleus and on the other 

 to the stapes or stirrup-bone, which is 

 connected by its base to the membrane 

 of the fenestra ovalis, which opens into 

 the internal ear. All these ossicles are 

 movable on each other, but they have no 

 lateral connection with any structure. 

 Sometimes at the end of the incus is found a separate bone known as the 

 os orbiculare. In the inner boundary of the middle ear is placed, in 

 addition to the oval window, a second, also communicating with the 

 vestibule, and called the fenestra rotunda, or round window. 



The internal ear, or labyrinth, is composed of bone, and consists of 



Fig. 403.— External Appearance 

 of the Labyrinth and Fe- 

 nestra Ovalis. {Landois.) 

 /upper, h horizontal, and s posterior semi- 

 circular canals of the loft side. The cochlea is 

 seen to the left of the figure. 



