EESPIEATORY SYSTEM. 



PHYSIOLOGICAL CONSIDERATIONS. 



Respiration and Structure of the Lungs. — 

 The organs of respiration are the larynx, 

 the upper opening of which is named glottis, 

 the trachea or windpipe, bronchia, and the 

 lungs. 



The air is displaced out of the lungs by 

 the action of the muscles of respkation; 

 and, when these relax, the lungs expand to 

 a certain calibre by their elasticity. This 

 may be exemplified by means of a sponge, 

 which may be compressed into a small bulk 

 by the hand, but, upon opening the same, 

 the sponge returns to its natm-al size, and 

 all its cavities become filled with air. The 

 purification of the blood in the lungs is of 

 vital importance, and indispensably neces- 

 sary to the due performance of all the func- 

 tions. "When the lungs, and muscles con- 

 nected with them, are in a physiological 

 state, the horse is said to be in good wind — 

 a very desirable state for an animal to be 

 in, whose usefiilness depends on his being 

 capable of a long continuance of quick 

 motion. The trachea, or windpipe, after 

 dividing into bronchia, again subdivides into 

 innumerable other branches, the extremities 

 of which compose an infinite quantity of 

 small cells, which, with the ramifications of 

 the veins, arteries, nerves, lymphatics, and 

 the connecting cellular membrane, make up 

 the whole mass or substance of the lungs. 

 The internal surface of the windpipe, bron- 

 chia, and air-cell, is lined with a membrane, 

 which secretes a mucous fluid: when, in con- 

 sequence of an obstructed surface, this fluid 

 becomes abundant, it is expelled by the 

 nostrils. The whole is invested with a 

 thin, transparent membrane, named pleura : 

 the same membrane lines the internal sur- 

 face of the ribs and diaphragm, and, by a 



duplicature of its folds, forms a separation 

 between the lobes of the lungs. 



RESPIRATORY SYSTEM. 



The function of respiration is the conver- 

 sion of venous into arterial blood. This 

 arterialization of the venous blood is a pro- 

 cess highly essential to the well-being of 

 all animals ; more important is it than the 

 assimulation of aliment; for a horse may 

 live several days without food, yet cannot 

 exist many minutes unless his blood be 

 arterialized. 



In considering the function of respiration, 

 our attention is first turned to the mechani- 

 cal means by which the air is alternately 

 admitted and discharged from the lungs. 

 The mechanical act of respiration is divisi- 

 ble into two periods, that of inspu-ation, 

 during which air is drawn into the lungs so 

 as to increase its volume and distend its 

 parenchema and expiration, during which 

 process the air wliich had been so received 

 is expelled. 



Inspiration is accompanied by enlarge- 

 ment of the capacity of the thorax in its 

 various dimensions. This is effected by the 

 action of different sets of muscles, operated 

 on by the nervous system. The principal 

 muscle of inspiration is the diaphragm. 



Among the secondary muscles employed 

 in inspiration are those which articulate the 

 ribs, viz., the intercostales. Each rib is 

 capable of a small degree of motion on the 

 extremity by which it is articulated with 

 the vertebrsB. This motion is chiefly for- 

 ward and backward ; the intercostal muscles 

 favor this motion, as they are disposed in 

 two layers, each passing obliquely, but with 

 opposite inclinations, from one to the adja- 

 cent rib. There are two ways in which the 



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