THE TRACHEA, OR WIXDPIPE. 259 



arclilike 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 

 Ihby are, to a great degree, protected by the surrounding parts ; and, 

 postei'iorly, they overlap each other, and the overlapping portions are con- 

 nected 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 por- 

 tions — 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 which prevents the arch from spurring out. In the 

 natural state of the windpipe this muscle is, probably, quiescent ; but 

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

 upper part by tight-reining, or at the lower end by an ill-made collar, or 

 anywhere by bi-utal or accidental violence, this muscle conti\acts, 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 injury or laceration. There are many beautiful points in 

 the physiology of the horse which deserve much greater attention than 

 has hitherto been paid to them. 



The windpipe should project from the neck. It should almost seem as 

 if it were detached from the neck, for two important reasons : first, that 

 jt may easily enter between the channels of the jaw, so that the horse may 

 be reined up without suffering inconvenience ; and next, that being more 

 loosely attached to the neck, it may more readily adapt itself to the changes 

 required than if it were enveloped by fat, or muscle to a certain degree 

 unyielding : therefore, in every well-formed neck — and it will be seen in 

 the cut (p. 237) — it is indispensable that the windpipe should be prominent 

 and loose on the neck. This is not required in the hea\'y cart-horse, and 

 we do not often find it, because he is not so much exposed to those cir- 

 cumstances that will hurry respiration, and require an enlargement in 

 the size of the principal air-tube. 



When the trachea arrives at the thorax, it suddenly alters its foi-m, in 

 order to adapt itseH" to the narrow triangular aperture through Avhich it 

 has to pass. It preserves the same cartilaginous structure ; for if it has 

 not the pressure of the external muscles, or of accidental violence, to resist, 

 it is exposed to the pressure of the lungs, when they are inflating, and it 

 shares in the pressure of the diaphragm, and of the intercostal muscles, in 

 the act of expiration. Having entered the chest, it passes a Httle to the 

 right, leaving the oesophagus, or g-ullet, on the left ; it separates from the 

 dorsal vertebrae ; it passes through the duplicatru'e of the mediastinum to 

 the base of the heart, and it divides beneath the posterior aorta. Its divi- 

 sions are called the hroiicliial hibes, and have much to do ■svith the well- 

 being of the horse. 



Its rings remain as perfect as before, but a new portion of cartilage 

 begins to present itself : it may be traced as high as the tenth ring from 

 the bottom ; it spreads over the union between the posterior terminations 

 of the rings ; it holds them in closer and firmer connection with each other ; 

 it discharges the duty of the transverse muscle, which begins here to dis- 

 appear, and the support of the cervical and dorsal vertebrae ; it prevents 

 the separation of the rings when the trachea is distended ; it spreads down 

 upon, and defends the commencement of the bronchial tubes. Some other 



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