PHYSIOLOGY. 11 



RESPIRATION. 



A function common to all animate beings, and by means 

 of which in animals, the venous blood mixed with the chyle 

 obtains its nutritive properties, under the influence of the 

 oxygen of the atmosphere. The atmosphere surrounds the 

 earth to the extent of from fourteen to fifteen leagues, and is 

 equal in weight to a similar bed of water about thirty-two 

 feet in thickness, or of a like one of mercury about twenty- 

 nine inches thick. This gas is formed of two elements 

 twenty-one being of oxygen, and seventy-nine of azote in a 

 hundred parts. Vapoury particles of water are also found in 

 the atmosphere, and traces of carbonic acid. This last gas 

 arises from the circumstance that animals, in the act of res- 

 piration, convert a certain quantity of oxygen into carbonic 

 acid. It is itself again decomposed by plants, which take to 

 themselves the carbon and give out the oxygen. The respi- 

 ratory apparatus is composed, in man, of the thorax, the tra- 

 chea and the lungs. 



Thorax. 



This is a cavity formed by the dorsal portion of the verte- 

 bral column behind, by the sternum before, and at the sides 

 by the ribs, with the intercostal muscles filling up the spaces 

 between them ; also by a muscle called the diaphragm, clo- 

 sing the cavity below, and by means of which its capacity 

 may be increased or diminished. 



Trachea, 



Or wind pipe, is a cylindrical canal, formed by the larynx, 

 whose upper opening is called the glottis, and by the trachea 

 proper, composed of little cartilaginous rings, connected by 

 membranous partitions. It terminates in the bronchice, which 

 are divisions of the trachea, leading one into each lung, of 

 which the right is the longer. 



Lungs 



Are spongy organs, enveloped by pleurce, and formed of the 

 bronchial divisions ending in a cul-de-sac among the pulmo- 

 nary cells, the ramifications of the pulmonary artery and 

 veins, and the cellular tissue uniting these diiferent organs so 

 as to form an infinity of small cells. 



