BREATHING. 277 



PHYSIOLOGY OF RESPIRATION. 



The essence of the act of breathing consists in the absorption 

 of oxygen from the air, and the excretion of carbonic acid from 

 the blood which is circulated through it. In a state of rest this 

 interchange must go on with regularity, for carbonic acid is con- 

 stantly developed by the decay of the tissues, arising from the 

 peculiar necessities of the muscular and nervous tissues, and by 

 the conversion of the carbon of the food which appears to be 

 required for the development of heat. But when the muscles 

 of the whole body are called into play with unusual rapidity and 

 force, the development of carbonic acid is largely augmented, and 

 thus, not only is there a necessity for extra means of excreting 

 the carbonic acid, but there is also a demand for more oxygen to 

 unite with the carbon, which is the result of the disintegration 

 of the muscular fibres employed. Hence the acts of respiration 

 are more complete and rapid during exercise than in a state of 

 rest, and while much more carbonic acid is given off, a greater 

 volume of oxygen is absorbed from the air which is inspired. 



It is pound by experiment that if venous blood is exposed 

 to the action of oxygen, through a thin membrane such as blad- 

 der, it absorbs a portion of that gas, and changes its color from 

 dark red to a bright scarlet. This is in accordance with the re- 

 cognised laws of endosmose and exosmose ; and as the blood cir- 

 culates in very fine streams within the vessels of the lungs, whose 

 walls are much thinner than an ordinary bladder, it may readily 

 be understood that it is placed in more favorable circumstances 

 for this interchange of gases than when tied up in a large mass 

 within a comparatively thick membrane. On examining the 

 structure of the lungs, they are found to be made up of a pair 

 of cellular sacs, communicating with the trachea, which admits 

 air into them ; and these sacs are furnished with a fine network 

 of capillary vessels distributed on their walls, and on those of the 

 numerous cellular partitions of which they are composed. Thus 

 the blood, as it enters the lungs in a venous state, is submitted 

 under very favorable circumstances to the agency of atmospheric 

 air j it readily absorbs the oxygen while it gives off large volumes 

 of carbonic acid gas, the result of the combination of previously 

 absorbed oxygen with the carbon given off by the various organs 

 of the body already allunded to. 



The exact chemical changes which have taken place in the 

 atmospheric air exhaled from the lungs and in the blood itself are 

 believed to be as follows: 1. A certain portion of oxygen has dis- 

 appeared from the air. 2. It has received a considerable volume 

 of carbonic acid. 3. It has absorbed fresh nitrogen. 4. It has 

 parted with some of the nitrogen of which it was previously made 

 up. The last two changes cannot readily be demonstrated, but are 

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