THE RESPIRATORY SYSTEM 



The respiratory apparatus (Apparatus respiratorius) comprises the nasal 

 cavity, 1 the pharjaix, 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 parts of the sj'stem are passages by which the air 

 passes to and from the lungs. The nasal cavity opens externally at the nostrils, 

 and communicates behind with the pharynx through the choanse or posterior nares; 

 it contains the peripheral part of the olfactorj^ apparatus, which mediates the sense 

 of smell. The pharjaix is a common passage for the air and food — a remnant of the 

 primitive embrj'ordc arrangement; it has been described with 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 di- 

 minish 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 thymus, are 

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

 torj' 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, enclosed 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 or choanae. 



The nostrils (Nares) are somewhat oval in outline, and are placed obliquely, 

 so that they are closer together below than above. They are bounded by two alse or 

 wings (Alse nasi), which meet above and below, forming the commissures. The 

 lateral ala is concave; the medial one is convex above, concave below. The 

 prominence of the medial ala is caused by the lamina of the alar cartilage; this 

 prominence, together with the alar fold which extends backward from it, partially 

 divides the nostril into a large lower part and a small upper part, which are pop- 

 ularly distinguished as the "true" and "false" nostril.^ The superior commissure 

 is narrow, the inferior one wide and rounded. If the finger is passed into the nostril 

 at the upper commissure, it enters a blind cutaneous pouch. This cul-de-sac, the 

 diverticulum nasi, extends backward to the angle of junction of the nasal bone and 

 the nasal process of the premaxilla. In order to enter the nasal cavity the finger 

 should be introduced at the inferior commissure and directed toward the septum 

 nasi. The naso-lacrimal orifice (Ostium nasolacrimale), the external opening of 



1 A nose fXasus extemus), such as exists in man, forming a projection distinctly marked 

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



^ These terms should be abandoned, as they tend unnecessarily to complicate the description 

 as well as to misconception with regard to function. 



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