EESPIEATION. 457 



expulsion by coughing of any noxious agent, and the protec- 

 tion of the lungs from injury. The muscles of the larynx are 

 supplied by the inferior laryngeal or recurrent nerve, a branch 

 given off inside the thorax by the pneumogastric. This nerve 

 is frequently affected in roaring. 



The windpipe or trachea is a flexible tube, slightly com- 

 pressed from above downwards, extending from the larynx 

 along the lower border of the neck, and within the thorax as 

 far as the right side of the posterior aorta, where it divides 

 into two bronchi. It is formed of a congeries of cartilaginous 

 rings, imperfect posteriorly, where the one end overlaps the 

 other, and showing their greatest breadth towards the middle 

 of the neck. About fifty of these rings are found in the 

 horse, and as many as seventy in ruminants. They are sub- 

 ject to singularities of form, sometimes becoming bifurcated 

 or joining each other at their extremities. Their adjacent 

 margins are bound together by yellow elastic tissue, which 

 extends over both their anterior and posterior aspects, 

 though in the thickest layer on the former. Their extremi- 

 ties, moreover, are bound together by this elastic layer, so 

 that the latter forms, with the cartilage, a complete external 

 coat to the tube. The extremities of the cartilages are 

 further joined by involuntary muscular fibres (trachealis 

 muscle), which are extended transversely on the inner aspect 

 of the elastic layer. Spread over the inner aspect of the 

 elastic and muscular fibres, is a layer of connective tissue 

 one-eighth of a line in thickness, and joining the former to 

 the mucous membrane. The mucous membrane is composed 

 of a connective and elastic layer, covered by ciliated epithe- 

 lium. This membrane, which is supplied by the recurrent 

 nerve, is possessed of very little sensibility. 



On arriving above, and to the right side of the heart, the 

 trachea divides into two larger bronchi, and in most of the 



