218 THE TRACHEA OR WINDPIPE. 



is possessed of very great sensibility, and its function requires it. It is, as hag 

 been already stated, the inner guard of the lungs, and the larynx must undergo 

 a multitude of 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, 

 however, 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. 113), passing along the anterior portion of the neck, and reaching 

 from the lower edge of the cricoid cartilage (11, p. Ill) 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 obstruction 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 com- 

 posed of cartilage, an exceedingly elastic substance, and at the same time pos- 

 sessing 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 suffi- 

 ciently 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 neces- 

 sary 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 pos- 

 sessing sufficient 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, eLtstic, and }et so strong 

 that it is scarcely possible to rupture it ; and the fibres of that ligament no\ 

 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. Laterally 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 stretch- 

 ing across the windpipe, dividing the canal into two unequal portions the 

 anterior one constituting the proper air-passage, and the posterior one occupied 



