,64 ANATOMY AND DISEASES OF THE NL:K. 



The .ining memurane is a continuation of \,hat of the pharynx above and Hit 

 trachea below. It is covered with innnmerable follicular glands, from whose mouths 

 there oozes a mucous fluid that rjioistens and lubricates its surface. It is possessed 

 of very great sensibility, and its function requires it. It is, as has been already 

 stated, the inner guard of the lungs, and the larynx must undergo a multitude oi 

 changes of form in order to adapt itself to certain changes in the act of respiration, 

 and in order to produce the voice. The voice of the horse is, how ever, 'extremely 

 limited, compared with that of the human being ; the same sensibility, therefore, is 

 not required, and exposed as our quadruped slaves are to absurd and barbarous usage, 

 too great sensibility of any part, and particularly of this, would be a curse to the 

 animal. 



THE TRACHEA OR WINDPIPE. 



The course of the inspired air from the larynx to the lungs is now to be traced, and 

 it will be found to be conveyed through a singularly constructed tube (6, p. 72), 

 passing along the anterior portion of the neck, and reaching from the lower edge of 

 the cricoid cartilage (11, p. 72) to the lungs. In the commencement of its course 

 it is somewhat superficially placed, but as it descends towards the thorax it becomes 

 gradually deeper and more concealed. In order to discharge its functions as an air- 

 tube, it is essential that it should always be pervious, or, at least, that any obstruc- 

 tion to the process of respiration should be but momentary. Attached to a part 

 endowed with such extensive motion as the neck, it is also necessary that it should 

 be flexible.. It is composed of cartilage, an exceedingly elastic substance, and at the 

 same time possessing a certain degree of flexibility. 



The windpipe is composed of cartilage, but not of one entire piece, for that would 

 necessarily be either too thick and firm to be flexible, or if it were sufficiently flexible 

 to accommodate itself to the action of the neck, it would be too weak to resist even 

 common pressure or injury, and the passage through it would often be inconveniently 

 or dangerously obstructed. Besides, it is necessary that this tube should occasionally 

 admit of elongation to a considerable degree. When the neck is extended in the act 

 of grazing or otherwise, the trachea must be lengthened. 



The structure of the cartilage of the windpipe is admirably adapted to effect every 

 purpose. It is divided into rings, fifty or fifty-two in number, each possessing suffi- 

 cient thickness and strength to resist ordinary pressure, and each constituting a joint 

 with the one above and below, and thus admitting of all the flexibility that could be 

 required. These rings are connected together by an interposed fibro-ligamentous 

 substance, extensible, elastic, and yet so strong that it is scarcely possible to rupture 

 it ; and the fibres of that ligament not running vertically from one to another, and 

 therefore admitting of little more motion than the rotation of the head, but composed 

 of two layers running obliquely, and in contrary directions, so as to adapt themselves 

 to every variety of motion. 



These rings are thickest in front, and project circularly, opposing an arch-like form. 

 There, too, the ligament is widest, in order to admit of the greatest motion in the 

 direction in which it is most needed, when the head is elevated or depressed. Late- 

 rally these rings are thinner, because they are, to a great degree, protected by the 

 surrounding parts ; and, posteriorly, they overlap each other, and the overlapping 

 portions are connected together by a strong ligamentous substance. This, while it 

 does not impede the motion of the tube, gives firmness and stability to it. 



Within the trachea is another very curious structure. At the points at which 

 posteriorly, the rings begin to bend inwardly, a muscle is found stretching across the 

 windpipe, dividing the canal into two unequal portions the anterior one constituting 

 the proper air-passage, and the posterior one occupied by cellular texture. It is to 

 give additional strength to parts. It is the tie w ich prevents the arch from spurring 

 out. In the natural state of the windpipe this muscle is, probably, quiescent; bill 

 when any considerable pressure is made on the crown of the arch at the upper part 

 by tight reining, or at the lower by an ill-made collar, or any where by brutal 01 

 accidental violence, this muscle contracts, every serious expansion or depression of 

 the arch is prevented, and the part is preserved from serious injury. 



It may also be readily imagined that, when in violent exertion, every part of the 

 respiratory canal is on the stretch, this band may preserve the windpipe from inju j 



