846 THE AFPABATUS OF TSE SENSES. 



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

 tympanic cavity, this renewal being indispensable to the perfect accomplish- 

 ment of hearing." — Lavocat. 



(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, and 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 ia inspiration.) 



Aetiole III. — The External Eae. 



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

 appendage opening outwardly, designated the concha or pavilion. 



THE EXTBBNAL AUDITORY CANAL. 



This canal (meatus auditorius externus), described in the osteology (as 

 situated in the petrous bone), has at the bottom the membrana tympani, 

 which separates it from the middle ear. Its entrance, the external auditory 

 hiatus, gives attachment to the infundibulum 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 eeruminous glands, as they secrete an unctuous matter, the cerumen. 



THE CONCHA OB 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 organisation : a cartilaginous frameworh composed of three 

 pieces, muscles to move these, an adipose cushion to insure liberty of 

 movement, and integuments covering the whole. 



1. Cartilages of the Concha. 



The three pieces composing the concha are : 1, The conchal cartilage ; 2, 

 Annular cartilage ; 3, Scutiform cartilage. 



1. Conchal Cartilage. — The principal portion of the pavilion, this 

 cartilage determines its general configuration. In shape, it • resembles a 

 trumpet with a wide opening on one side. Its entrance is elliptical, and 

 elongated vertically, being circumscribed by two thin borders which unite 

 above at a point that constitutes the summit (apex) of the organ. Its base, 

 bulging in a cul-de-sac, terminates in front by a constricted infundibulum ; 

 it is attached to the margin of the auditory hiatus by means of the annular 

 cartilage, and to the surface of the guttural pouch by a pointed prolongation 

 that descends outside this annular cartilage, beneath the parotid gland, and 

 terminates by several fibrous filaments. This portion of the framework of 

 the concha is a cartilaginous plate, rolled on itself in such a manner as to 

 circumscribe, between its borders, the entrance to the ear, and to form in- 

 feriorly, the complete infundibuliform canal just mentioned. In Solip'eds, 

 this plate is rigid and erect, and much more developed in the Ass and Mule 

 than in the Horse. (Externally, the integument of the ear is covered by 

 ordinary hair, but internally there are long fine hairs, especially near the 

 entrance ; these prevent the intrusion of foreign substances.) 



2. Annular Cartilage — By this name is known a little ring-shaped 



