844 STRUCTURE OF THE SHEEP. 



auricles, at any rate that part of it situated behind the vah-es. 

 By these means an agitation is produced which effectually 

 mixes these different fluids together. 



It has been ascertained that the veins possess a power of 

 absorption in common with a numerous class of vessels 

 called the absorbents, or lymphatics. These vessels are 

 very minute, and are distributed throughout the whole body ; 

 they generally accompany the veins, and, like them, are fur- 

 nished with valves. 



0\ RESPIRATION AND ITS EFFECTS. 



The phenomenon of respiration, which is carried on from 

 the first mirmte after birth to the last of existence, consists 

 of two acts, inspiration and expiration. The former, that of 

 inhaling the atmosphere, is accomplished mostly by the 

 diaphragm, which, in its relaxed state, is convex towards 

 the chest. As its fibres contract, the muscle flattens, and 

 thus enlarges in a considerable degree the cavity of the 

 thorax. A vacuum is thus produced, or rather a tendency 

 towards it; for the air rushes into the lungs, and the blood 

 into the heart ; and, as the lungs are elastic and spongy in 

 their nature, they become closely adapted to the enlargement 

 of the chest, and prevent any vacuum from taking place be- 

 tween them and the sides of the thorax. The diaphragm is 

 thus the chief agent in the act of inspiration, although in 

 some degree assisted by the intercostal muscles, which raise 

 the chest, and also, when the breathing is violently excited, 

 by those muscles that in quadrupeds attach the fore extremi- 

 ties to the body. The air thus drawn into the lungs traverses 

 throughout its internal surface, and, having fulfdled its office, 

 is forced out by the act of expiration. This part of the 

 process is effected chiefly by means of the elasticity of the 

 lungs, which acts as soon as the diaphragm becomes passive, 

 assisted, however, in some degree by the elastic cartilages 

 of the chest, and occasionally by the abdominal muscles. 



Atmospheric air consists of unequal parts of two aeriform 

 fluids, viz., four-fifths of nitrogen or azote, and one-fifth of 

 oxygen in each 100 parts ; besides which it contains other 

 heterogeneous matters, such as odorous effluvia, aqueous ex- 

 halations, 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 that 

 of others a much greater height. Its gravity differs very 



