THE RESPIRATORY SYSTEM 



The orp;ans of respiration (Apparatus respiratorius) comprise the nasal cavity/ 

 the pharynx, the larynx, the trachea, the bronchi, and the lungs. The lungs are the 

 central organs in which the exchange of gases between the blood and the air takes 

 place; the other ]oarts of the system are passages by which the air passes to and 

 from the lungs. The nasal cavity opens externally at the nostrils, and communi- 

 cates behind with the i^harynx through the posterior nares (Choanae) ; it contains 

 the peripheral part of the olfactory apparatus, which mediates the sense of smell. 

 The pharynx is a common passage for the air and food — a remnant of the primitive 

 embryonic arrangement; it has been described as a part of the digestive tube. 

 The larynx is a complex valvular apparatus which regulates the volume of air pass- 

 ing through the tract; it is also the chief organ of voice. The trachea, and the 

 bronchi formed by its bifurcation, are permanently open conducting tubes. The 

 thorax, the pleural sacs which it contains, and the muscles which increase or 

 diminish the size of the cavity are also parts of the system. The bones, joints, 

 and muscles of the thorax have already been described. 



For topographic reasons two ductless glands, the thyroid and the thjmaus, are 

 usually described in this section, although they are in no sense a part of the respira- 

 tory system. 



RESPIRATORY SYSTEM OF THE HORSE 



THE NASAL CAVITY 



The nasal cavity (Cavum nasi), the first segment of the respiratory tract, is a 

 long, somewhat cylindrical passage, inclosed by all the facial bones except the 

 mandible and hyoid. It is separated from the mouth ventrally by the palate. 

 It opens externally at the nostrils, and communicates posteriorly with the pharynx 

 through the posterior nares. 



The nostrils or anterior nares (Nares) are somewhat oval in outline, and are 

 placed obliquely, so that they are closer together below than above. They are 

 bounded by two alae or wings (Alae nasi), which meet above and below, forming the 

 commissures. The outer ala is concave; the inner one is convex above, concave 

 l)elow. The upi)er commissure is narrow, the lower one wide and rounded. If 

 the finger is passed into the nostril at the upjxT commissure, it enters the so-called 

 false nostril, which leads backward to a blind cutaneous pouch. This cul-de-sac, 

 the diverticulum nasi, extends backward to the angle of junction of the nasal })one 

 and the nasal i)r()cess of the prcmaxilla. The posterior part of the diverticulum 

 is blind, but the false nostril communicates Ix^low and intenially with the vestibule 

 of the nasal cavity. The external opening of the naso-lacrimal duct is seen when 

 the nostril is dilated; it is situated on the floor of the vestibule, perforating the 

 skin close to its junction with the mucous membrane, about two inches (ca. 5 cm.) 

 from the lower commissure. (It is not rare to find one or two accessory orifices 

 furtli(>r back.) 



Structure. — The skin around the nostrils presents long tactile hairs as well as 

 the ordinary ones. It is continued around the alae and lines the vestibule. In the 



' An external nose (Nasus externus), sueh as exists in man, forming a projection distinctly 

 marked off from the rest of the face, does not exist in the domesticated animals. 



436 



