58 STRUCTURE AND ECONOMY OF THE HORSE. 



in some degree, by tlie elastic cartilages of the chest, and occa- 

 sionally by the abdominal muscles. In a healthy horse, in a 

 state of rest, there are from four to eight respirations in a minute, 

 which, however, are increased by the slightest exercise. 



Atmospheric air consists of unequal parts of two aeriform 

 fluids; viz., 79 of nitrogen or azote, and 21 of oxygen in each 

 100 parts (which are not chemically but mechanically com- 

 bined) ; besides which it contains other heterogeneous matters, 

 such as odorous effluvia, aqueous exhalations, electric matter, 

 and carbonic acid gas. It every where surrounds and embraces 

 the globe, extending, in the opinion of some, a distance of forty- 

 five miles, and in others a much greater height. Its gravity 

 differs very much at different times and in different places, 

 being heavier on a clear than on a close day, and also in low 

 places than in lofty ones. The small portion of carbonic acid 

 gas which the atmosphere contains, is mechanically mixed with 

 it. This gas is evolved by the fermentation of beer, and the 

 decomposition of vegetables, and is often found in wells and deep 

 places. It is much heavier than the atmosphere, and thus 

 remains in these low places by its gravity. A lighted candle 

 placed in this gas is immediately extinguished; so that it is used 

 as a safeguard in descending into these low and foul places, for 

 whatever will not support combustion will not support life. It 

 is not a simple gas like oxygen, but is formed by the union of 

 carbon and oxygen. 



Nitrogen or azote is a simple gas, but its use in the atmosphere 

 seems to be of a passive nature, being for the purpose of diluting 

 the oxygen, and rendering it less stimulating. It will not alone 

 support either life or combustion, but it is chemically mixed 

 with the oxygen. 



Oxygen is essential to the support of both life and combustion; 

 for if air be deprived of it no animal can live, nor will a candle 

 remain lighted. It is abundantly furnished by plants and shrubs, 

 which thus counteract the diminution of it created by animals. 

 When a flame is exposed to this gas it greatly increases in bril- 

 liancy ; and when venous blood is submitted to it, it quickly 

 becomes florid. 



We have before shown, that all the blood in the body was 

 in its turn carried from the heart to the lungs by means of 

 the pulmonary artery ; this divides and subdivides into the 

 smallest branches, and terminates in small capillary veins, which, 

 coalescing, become larger, and convey the blood again to the 

 heart by the pulmonary veins. Before it reaches these veins, how- 

 ever, an important change takes place : the blood proceeds from 

 the heart in a black and impure state ; it returns reddened and 

 purified ; it is submitted in its course to the action of the air in 

 the air cells, not by actual contact, but through the membrane 



