14 



from the heart, and are liable to much variation from the state 

 of the surrounding atmosphere. 



The production of animal heat is due to certain chemical and 

 "satal changes which are continually taking place in the body, and 

 consist in the absorption of oxygen by the capillaries in the lungs, 

 the combination of that oxygen with the carbon and hydrogen 

 derived from the disintegration of animal tissues, and from cer- 

 tain elements of the food which have not been converted into 

 tissue. 



This combination of oxygen, or oxidation, not only takes i)lace 

 in the blood, which may be looked upon as a fluid tissue, but in 

 the tissue cells also, in all parts of the body, the animal heat 

 being maintained by the natural changes which are essential to 

 a healthy condition. 



The lungs of a horse will contain from 1 to 1}^ cubic feet of 

 air, and at each inspiration about 250 cubic inches are drawn 

 through the trachea. The surface of the lungs to which this 

 amount of air is exposed is calculated to be equal to 289 square 

 feet. Air, then, containing impurities, is exposed to an absorb- 

 ent area within the body equal to five and a half times the sur- 

 face of the skin. A horse in a state of quiescence gives off from 

 2 to 3 cubic feet of CO- every hour from the lungs, and a certain, 

 though an undetermined, portion is also given off by the skin; 

 but taking that from the lungs alone it would amount to from 

 48 to 72 cubic feet in twenty-four hours, or, if converted into car- 

 bon, would give us a solid block weighing about 2^4 pounds. 



The air from marshes contains an excess of carbonic acid gas 

 and a diminished proportion of oxygen, consequently horses 

 should not be kept in the vicinity of a marsh, as the blood would 

 then contain an excess of carbonic acid gas (CO-) and an insufii- 

 cient amount of oxygen; it would be imperfectly purified; the 

 vitality of the animal would be lessened, and he would be more 

 susceptible and succumb more easily to any disease that he might 

 contract. 



A pure air will contain 20.99 per cent of oxygen; an average 

 air 20.96 x>er cent. The expired air, in health, in 100 parts con- 

 tains 19 instead of 21 parts of oxygen, and 1 to 2 per cent, instead 

 of 0.03 to 0.04 per cent, of carbonic acid gas, which pure air should 

 contain. 



