38 PHYSIOLOGY. 



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 respiration ; and when these relax, the lungs ex- 

 pand 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 hand, the sponge returns to 

 its natural size, and all its cavities become filled with air. 

 The purification of the blood in the lungs is of vital impor- 

 tance, and indispensably necessary to the due performance of 

 all the functions. When the lungs, and muscles connected 

 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 usefulness depends on his being capable of a long 

 continuance of quick motion. The trachea, or windpipe, 

 after dividing into bronchia, again subdivides into innumera- 

 ble 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, bronchia, 

 and air-cell, is lined with a membrane, which secretes a 

 mucous fluid : when, in consequence of an obstructed sur- 

 face, this fluid becomes abundant, it is expelled by the nos- 

 trils. The whole is invested with a thin, transparent mem- 

 brane, named pleura : the same membrane lines the internal 

 surface of the ribs and diaphragm, and, by a duplicature of 

 its folds, forms a separation between the lobes of the lungs. 



Glandular Secretions. — Liver. This is the largest gland 

 in the animal economy; it secretes from the blood a fluid 

 called bile ; it is conveyed by numerous small tubes into a 

 larger one, in which they terminate : this is named hepatic, 

 or biliary duct. In the human body, and in most quadrupeds, 

 there is another duct, branching off from this, which termi- 

 nates in a gall bladder, from which the bile is expelled ; but 



