114 PRINCIPLES OF VETERINARY SCIENCE 



and is the direct communication between the nostrils and the 

 pharynx. 



The pharynx has been described with the organs of the digestive 

 system, as it is common to both the digestive and the respiratory 

 tracts. 



The larynx is a short tube-like apparatus situated between the 

 pharynx and the anterior end of the trachea. It regulates the 

 amount of air passing to and from the lungs and prevents the 

 aspiration of foreign bodies. It is made up of five pieces of car- 

 tilage — the cricoid, the thyroid, and the epiglottis, which are 

 single, and the arytenoids, which are paired. These cartilages 

 articulate by true joints, and are moved by means of two systems 

 of muscles — (1) the extrinsic group, which finds one attachment to 

 the larynx and the other to some point outside the organ ; (2) the 

 intrinsic group, which is attached to the different cartilages. 

 The interior of the larynx shows two vocal cords stretched from 

 the lateral walls; the opening between these cords is called the 

 glottis. When the vocal cords vibrate, sounds are produced. 

 On each side of the larynx is a pocket-like depression of mucous 

 membrane, known as the ventricle. 



The trachea or windpipe is the tube that connects the larynx 

 with the lungs. It is formed of fifty to fifty-five rings of cartilage, 

 attached to each other by a fibro-elastic membrane. The rings 

 are not complete circles, for their two ends overlap superiorly. 

 Through these rings the organ is kept permanently open and is 

 made flexible. The average caliber is about 2% inches. Its 

 average length is about 31 inches. At the entrance to the lungs 

 the trachea divides to form the bronchi. 



The bronchi are two in number, the right being the larger. 

 Together with the pulmonary vessels they form the "root of the 

 lung." They are continued posteriorly by the smaller bronchial 

 tubes, which ramify throughout the lungs. 



The lungs are designated right and left. Together with the 

 heart they practically fill the thoracic cavity. Three systems of 

 tubes embedded in connective tissue are found in the lungs: (1) 

 the ramifications of the bronchial tubes; (2) the pulmonary 

 arteries; (3) the pulmonary veins. For description, each lung 

 may be considered as having a base, an apex, and two surfaces. 

 The base is situated against the diaphragm, which conforms to its 

 shape. The apex lies apposite the first and second ribs. The 

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