CHANGES ON THE AIR IN RESPIRATION. 255 



is offered, is the amount of mechanical effort required to 

 enlarge the chest. 



Such is the mechanism of respiration : the diaphragm de- 

 scends in man, or, to include animals that are prone, is thrown 

 towards the abdominal cavity, and the ribs are drawn nearer to 

 a right angle with the spine, by which changes the thorax is 

 enlarged and the air enters the lung this is inspiration : by an 

 opposite movement, which to a certain extent is spontaneous, 

 the chest is contracted in its dimensions, and the air in the 

 lung, being condensed, issues forth. 



The air which enters is the air of the surrounding atmos- 

 phere, consisting of one volume of oxygen, nearly four volumes 

 of nitrogen, and variable proportions of carbonic acid gas and 

 watery vapour. The air which issues from the lung has lost a 

 large proportion of its oxygen, has acquired a considerable pro- 

 portion of carbonic acid, and is charged with watery vapour. It 

 has often been contended that the proportion of carbonic acid 

 gas given off in expiration corresponds exactly in volume with 

 that of the oxygen consumed ; but subsequent inquiries show a 

 great variety in this respect under different circumstances, and 

 that as a rule much more oxygen is consumed than is required 

 to produce the carbonic acid gas thrown off. In short, the 

 oxygen consumed in respiration must suffice not only for the 

 oxidation of the carbon to be eliminated, but also for the oxi- 

 dation of the hydrogen, and of every other oxidisable substance 

 that is to be thrown off from the system, the only probable 

 deduction required being the amount of oxygen that may be 

 swallowed in the air mingled with the food after mastication. 



Excretion of Urine. The purification of the blood is in 

 particular ascribed to the agency of three important organs 

 namely, the lung, the liver, and the kidney. The lung 

 separates carbon in the shape of carbonic acid, the liver com- 

 pounds abounding in hydrogen, and the kidney urea, which is 



