NASAL CAVITIES 469 



MECHANISM OF THE PULMONARY APPARATUS 



Although the whole of these parts are not contained within the 

 thorax, it will be convenient to examine them together, since they all 

 mutually bear upon each other both in health and disease. 



The pulmonary apparatus of the horse consists of four parts — First, 

 the nasal cavities, destined to prepare the air for entering the larynx ; 

 second, of the larynx, which acts as a portal or guard against the admission 

 of noxious matters floating in it ; third, of a set of tubes, consisting of the 

 trachea and bronchi, which convey the air from the larynx to the aii'-cells ; 

 and fourthly, of the air-cells themselves, where the changes are effected in 

 the blood, for which the lungs are specially designed. 



THE NASAL ORIFICES AND CAVITIES 



The nasal orifices in the horse and ass differ from those of the other 

 domestic animals, and also from the human nostrils, in being the sole means 

 of admitting air to the lungs. The ox, sheep, dog, cat, etc. can breathe 

 either through the nostrils or the mouth, but the horse is prevented, by the 

 formation of his soft palate, from drawing in air through his mouth, and 

 hence he requires nostrils of a size calculated to admit an extra supply of 

 air.-*- The 07'ifices or nostrils consist of an oblong opening on each side of the 

 nose, separated from each other externally by the skin covering the carti- 

 laginous alee, which encircle three-fourths of the opening. These alse, 

 together with the septum, which divides the two nostrils vertically, con- 

 stitute the five cartilages of the nose, all being lined by the Schneiderian 

 membrane, upon which the nerves of smell are freely disti'ibuted. Each 

 nostril has two flexible and easily dilated alse — a superior or internal ala, 

 having a broad cartilaginous plate, and an inferior ala, of a crescentic 

 shape, with its concavity tui^ned inwards. The four al?e, when in position, 

 resemble the letter X, and form a framework which keeps the openings 

 always patulous, while it resists the actions of the muscles when they draw 

 the external folds of the skin away from it in order to enlarge the openings. 

 Two little pouches of skin are found internally above the true nostrils, and 

 ai^e called the false nostrils, the use of which is not clearly demonstrated. 

 At the inner and inferior part of the nasal fossa, underneath the fold of 

 skin covering the inferior ala, is the orifice of the nasal duct, which leads 

 down from the eye, and conveys the surplus secretion of lachrymal fluid 

 from that organ to the nose. In the ass and mule this orifice is found just 

 within the superior ala. If these alse are not of full size and the nostrils 

 patulous, it may generally be surmised that the other organs of respiration 

 are equally undeveloped, and that the horse's wind will be proportionally 

 bad. 



The nasal cavities, or foss^, are partly bounded by bone, and partly 

 by the cartilage known as the se2Jtuin nasi. The sui-face of membrane is 

 much increased by the convolutions of the turbinated bones, so that 

 the air, as it passes through these chambers, is warmed if cold, and if 



^ It is often observed tliat horses breathe through an open mouth shortly before they die. 



