CHEMICAL PHENOMENA OF RESPIRATION. 317 



mental curiosity than as an important physiological fact, the 

 influence, in a contrary direction, which may be observed to 

 exist between the heart and the lungs. "We know that the 

 pulsation of the heart changes the condition of the intra- 

 thoracic pressure ; supposing the thorax to be immovable, the 

 afflux of blood which takes place at each diastole, should 

 compress the air in the lungs, and, if the glottis is open, give 

 rise to a slight expiration ; in the same manner, when the heart 

 is suddenly emptied, the blood which gushes out of the thorax 

 is replaced by a certain quantity of air which enters through 

 the trachea. In the normal condition, we are scarcely sen- 

 sible of this, on account of the constant modifications pro- 

 duced by respiration in the respiratory capacity of the thorax. 

 The fact, however, can easily be made plain, by placing the 

 trachea of a dog in communication with the registering ap- 

 paratus, and then puncturing or severing the medulla 

 oblongata of the animal by a single stroke : respiration ceases 

 immediately, while the heart continues to beat for some 

 minutes, its pulsations being registered through the medium 

 of the air in the trachea " (P. Bert). 



IV. CHEMICAL PHENOMENA OF RESPIRATION. 



WE understand how the air and the blood are brought into 

 contact with each other, and also by what mechanism they 

 are constantly renewed ; we have now to examine the gas- 

 eous exchanges which are produced by this contact taking 

 place in the lungs : what these are we shall see by ascertain- 

 ing the changes made in the air and in the blood, during their 

 passage through the lungs. 



A. Modifications in the air exhaled. 



We know that 10 cubic metres (10,000 litres) of air are 

 received into the lungs daily, and that nearly an equal quan- 

 tity is expelled : we thus retain about ^ or -fa of the air 

 inhaled ; at the first examination, however, the exhaled gas 

 is found undiminished in quantity, on account of the vapor 

 contained in it, which occupies a considerable space. A still 

 more important change which takes place in the air is the 

 loss of oxygen, which is replaced in a great measure by car- 

 bonic acid, one-fifth of the amount of the 10 cubic metres of 

 air inhaled is oxygen (21 parts of O. to 79 parts of N.) ; this 

 is equal by weight to 2J kilos, of oxygen. In the air ex- 



