178 ANATOMY OF THE HEAD. 



common skin ; which, however, in this part assumes an 

 altered appearance, and becomes still more fine and attenu- 

 ated as it lines the meatus auditorius, across the bottom of 

 which it is stretched. This skin has upon it several fol- 

 licles, to secrete the w^ax of the ear. The membrana 

 tympani is the covering of a cavity called the tympanum 

 of the ear; over which this delicate membrane is expanded, 

 and by which the inner is separated from the outer ear. 

 The tympanum is a portion of the internal cavity, being 

 irregularly spherical. It contains four small bones, which 

 are named, according to their supposed resemblances, 

 incus, malleus, stapes, and orbicularis ; by whose move- 

 ments it is conjectured the impressions received by the 

 membrana tympani are regulated ; and which impressions 

 receive further modifications within the more interior parts. 

 These bones are moved by muscles, which it w^ould here be 

 a waste of time to describe, as the ear of the horse is beyond 

 those delicate remedies by which deafness is relieved. The 

 internal ear presents several openings ; as those of the 

 mastoid cells, the Eustachian tubes, and the communication 

 between the cavity and the labyrinth. The mastoid cells 

 are small irregular cavities in the substance of the mastoid 

 processes, of the petrous temporal bone ; fined by a fine 

 membrane, and communicating with each other, having a 

 common entrance near the Eustachian canal. The Eusta- 

 chian tube is an opening at the upper and anterior edge of 

 the hollow of the tympanum, forming a duct wdiich is in 

 part bony, and in part cartilaginous ; extending from the 

 tympanum to the guttural pouches {Plate Yll. j), at the 

 posterior part of the nasal cavity. 



The guttural pouches {Plate VII. j) are large and empty 

 sacs, whose use is not known ; and whose size is by no 

 means equalled in other quadrupeds : but in the horse they 

 are opposed to each other, being connected only by trans- 

 parent cellular tissue. Each is closed by a valvular open- 

 ing, which separates it from the nose ; and each seems 

 to be nothing more than a bladder of mucous membrane 

 placed within the centre of the horse's head. They may 

 have some influence upon the voice ; but the principal ser- 

 vice to us at present appears to be to fill up a space wdiich 

 else would be left vacant, and thus to keep up that beauty 



