?58 THE TRACKEA, OR WINDPIPE. 



Roinewliat complicated system of muscles, for a description of wliicli tlie 

 reader is referred to the Sth vol. of ' The Veterinarian,' p. 447. The 

 entire process of respu-ation is partly under the control of the "will, and 

 the muscles of the larynx concerned in one stage of it are likewise so, but 

 they also act independently of the will, for during sleep and unconsciousness 

 the machine continues to work. 



The origin of the artery which supplies these parts with blood is some- 

 times derived from the main trunk of the carotid, but oftener it is a 

 branch of the thyroideal arter}'. 



The lining membrane is a continuation of that of the pharynx above 

 and the trachea below. It is covered with innumerable follicular glands, 

 irom whose mouths there oozes a mucoas fluid that moistens and lubricates 

 its surface. It is possessed of very great sensibility, which is derived from 

 the superior laryngeal nerve, and its fonction requires it. It is, as has 

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

 Lindergo a multitude of changes of form in order to adapt itself to certain 

 changes in the act of respiration, and in order to pi^oduce the voice. The 

 voice of the hvirse is, however, extremely limited, compared with that of 

 the human bemg ; 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 con- 

 structed tube, passing along the anterior portion of the neck, and reaching 

 from the lower edge of the cricoid cartilage to the lungs. In the com- 

 mencement of its course it is somewhat superficially placed, but as it 

 descends towards the thorax it becomes gradually deeper, and more con- 

 cealed. 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 composed of cai'tilage, an exceedingly elastic 

 substance, and at the same time jDOSsessing a certain degree of flexibility. 



The windpipe is composed of cartilage, but not of one entire 23iece, for 

 that Avould 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 ob- 

 structed. 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 sufficient thickness and strength to resist ordinary pressure, 

 and each constituting a junction 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, exten- 

 sible, elastic, and yet so strong that it is scarcely possible to ruptui-e 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 circulai-ly, opposing an 



