THE AUDITORY APPARATUS. ■ 957 



of the stylo-hyoideus, etc., it is smooth internally, and lubriiied by the mucus it 

 secretes. It may become the seat of purulent collections, which compress the 

 larynx and obstruct the respiration, and it is in such cases that the pouch is 

 punctured.^ 



This membrane receives numerous fine vascular and nervous ramifications 

 from the neighbouring branches. 



The guttural pouches communicate with the pharynx and cavity of the 

 tympanum, and usually contain air ; the quantity of this may vary in health, 

 according to the degree of dilatation of these membranous sacs. Their dilatation 

 is chiefly produced by the palato-pharyngeus muscle, several fibres from which 

 extend to their mucous membrane ; and, besides, when the ear is erected this 

 membrane is thrown into a state of tension, through the adhesion of the lower 

 prolongation of the concha to its surface. 



The functions of the guttural pouches are far from being well known. It 

 cannot be affirmed that they increase phonation ; indeed, their use appears to be 

 rather related to audition, if it be considered that these annexes of the guttural 

 duct of the tympanum coincide, in Sohpeds, with a less development of the 

 mastoid cells than in the other animals. 



With regard to the Eustachian tube, it serves to renew the air in the tympanic 

 cavity, this renewal being indispensable to the perfect accomplishment of hearing. 



The epithelium lining the guttural pouches is cylindrical and vibratile. 



(It is essential that the equilibrium between the external air and that in the 

 cavity of the tympanum should be maintained, in order to avert irregular tension, 

 or even rupture, of the membrana tympani. Perosino states that the guttural 

 pouches are filled with warm air during expiration, and that this is partly changed 

 for cold air in inspiration.) 



Aeticle III. — The Exteenal Eae. 



The external ear comprises the external auditory canal, and a widened appen- 

 dage opening outwardly, designated the concha, or imvilion. 



The External Auditory Canal. 



This canal {meatus auditorius externus), described in the Osteology, is, in the 

 Horse, of a cylindro-conical shape, and has an average width of '020 to '022 m. 

 It has at the bottom the membrana tympani, which separates it from the middle 

 ear. Its axis forms with the surface of that membrane an angle of about 30°. 

 Its entrance — the external auditory hiatus — gives attachment to the infundi- 

 bulum of the conchal apparatus. It is lined by a thin integumentary membrane, 

 intermediate in character between the skin and mucous membrane, and has 

 in its substance a large number of glands and convoluted tubes, analogous to 

 the sudoriparous glands — but here named ceruminous glands, as they secrete an 

 unctuous matter — the cerumen. 



The Concha, or Pavilion. 



The external trumpet-shaped appendage named the concha (concha auris) 

 varies much in shape in the different animals, though in all it offers the same 

 details in organization — a cartilaginous framework composed of three pieces, 



' Bartheleray, sen., and Goubaux, have found, in the guttural pouches, solid masses formed 

 of mucus and epithelial cells. 

 63 



