RESPIRATORY SYSTEM. 223 



width gradually diminishes from the middle towards either ex- 

 tremity, and bears a ratio of about one to six when compared to 

 its lengtli. The sides turned forwards, are formed by the aryte- 

 noid cartilages ; those directed backwards, by two prominent 

 folds of membrane (which envelope the thyro-arytenoid mus- 

 cles), commonly described as the vocal ligaments, from their being 

 concerned in the formation and intonation of the voice. Imme- 

 diately over them are slit-like apertures, opening into membranous 

 sacs, each large enough to contain a walnut ; these are the ven- 

 tricles of the lon/iix, whose use is also connected with the pro- 

 duction and modulation of the voice. 



The membrane lining the cavity of the larynx is one of great 

 susceptibility ; on which account it is kept continually moist by a 

 mucus, oozing from numerous lacuncz — the excretory orifices of 

 small subjacent follicles whose situation is denoted by the little 

 round eminences upon its surface. This is the common seat of 

 that species of catarrh which is accompanied by cough. 



OF THE TRACHEA. 



The trachea, or windpipe, is a cartilaginous tube extending 

 along the neck, from the larynx to the lungs, for the passage of 

 air. In horses of ordinary size, it is from twenty-five to thirty 

 inches in length. 



Course. — The trachea commences from the inferior border of the 

 cricoid cartilage, opposite to the body and transverse processes of 

 the atlas ; takes its course along the anterior and inferior part of the 

 neck, inclining to the near side, between the sterno-myloidei mus- 

 cles (which by their approximation conceal the lower portion of 

 it), and enters the chest between the two first ribs ; wherein, under 

 the curvature of the posterior aorta, it divides into two parts, the 

 bronchial tubes. 



Structure. — From fifty to sixty annular pieces of cartilage enter 

 into the composition of the windpipe; altogether constituting a 

 structure so remarkable for the inequality or asperity of its exterior, 

 that the ancients, in order to at oncedistinguish it from all other ves- 

 sels, called it the asp ('/a arteria. No entire or undivided tubular 

 substance could have partaken of the various motions of the head 

 and neck, without having suffered more or less distortion, and con- 

 sequent deformity and cHminution of caliber, of some part of its 

 canal, which would have been attended with frequent interruptions 

 to the free passage of the air, dangerous, and even fatal, to the re- 

 spiratory functions; whereas, constructed as it is, with the aid of 

 its muscular power, no attitude into which the animal may natu- 

 rally put himself will impede the freedom of passage through it. 

 The cartilages, or, as they are commonly described, the rings of 



