TRACHEOTOMY. 219 



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 wind- 

 pipe 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 by an ill -made collar, or anywhere by brutal or 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 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 it 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. 212) it is indispensable 

 that the windpipe should be prominent and loose on the neck. This is not 

 required in the heavy cart-horse, and we do not often find it, because he is not 

 so much exposed to those circumstances which 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 form, in order 

 to adapt itself to the narrow triangular aperture through which 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 little to the right, leaving the oesophagus, or gullet, 

 on the left ; it separates from the dorsal vertebrae ; it passes through the dupli- 

 cature of the mediastinum to the base of the heart, and it divides beneath the 

 posterior aorta. Its divisions are called the bronchial tubes, and have much to 

 do with 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 connexion with each other ; it discharges the duty of 

 the transverse muscle, which begins here to disappear, 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 small plates of cartilage reach a considerable 

 way down the divisions of the bronchi, and the last ring has a central triangular 

 projection, which covers and defends the bifurcation of the trachea. 



TRACHEOTOMY. 



The respiratory canal is occasionally obstructed, to an annoying and dangerous 

 degree. Polypi have bem described as occupying the nostrils ; long tumours 

 have formed in them. Tumours of other kinds have pressed into the pharynx. 

 The tumour of strangles has, for a while, occupied the passage. The larynx 

 has been distorted ; the membrane of the windpipe, on the larynx, has been 

 thickened, and ulcers have formed in one or both, and have been so painful that 

 the act of breathing was laborious and torturing. In all these cases it has been 



