THE RESPIRATORY SYSTEM 
The respiratory apparatus (Apparatus respiratorius) comprises the nasal 
eavity,! 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 parts of the system 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 choanz or posterior nares; 
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 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- 
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, 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 choane. 
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 ale or | 
wings (Ale 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.2. 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 0! 
| 
1A nose (Nasus externus), such as exists in man, forming a projection distinctly marked 
off from the rest of the face, does not exist in the domesticated animals. 
2 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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