CHAP, vm.] ORGANS OF RESPIRATION AND SECRETION. 221 



gives out oxygen and takes in carbonic acid. Internal respiration, 

 therefore, is the absorption of oxygen and the elimination of carbonic 

 acid by the ultimate parenchyma of the body's substance, which is 

 bathed by the nutritious oxygenated fluid as it exudes from the 

 capillary vessels. External respiration is the absorption of oxygen, 

 and the elimination of carbonic acid by the blood, on what is 

 essentially the surface of the body ; for the lining of the lungs is but, 

 as it were, a very deep and complex inbending and infolding of the 

 body's external surface, as has been already pointed out in the 

 second chapter. The oxygen thus received in the lungs, appears in 

 part to form an actual chemical union with the matter of the red 

 corpuscles, and in part to be dissolved in the liquor sanguinis. The 

 elimination of carbonic acid, it is maintained, is produced by an 

 actual process of chemical decomposition. 



It is necessary that the air respired be more or less fresh, and it 

 cannot be used over and over again an indefinite number of times ; 

 for all that some oxygen still remains within it. 



Ordinary atmospheric air contains nearly 7,900 parts of nitrogen 

 and 2,100 of oxygen, with a small quantity (3 parts in 10,000) of 

 carbonic acid. 



The air expired, however, has about 470 parts of carbonic acid, 

 and less than 1,600 parts of oxygen the quantity of nitrogen 

 remaining about the same as in fresh air. Thus about 5 per cent, 

 of oxygen is gained, and 5 per cent, of carbonic acid is lost in the 

 process of respiration. These changes correspond with changes in 

 the blood. Thus, in 100 parts of venous blood there may be five 

 parts of oxygen and twenty-five of carbonic acid, while in the same 

 quantity of arterial blood there may be ten parts of oxygen and 

 twenty of carbonic acid. 



However rich in oxygen the air inspired may be, no more oxygen 

 is absorbed than the processes of life demand. The organism 

 regulates itself in this respect. The more the blood is charged with 

 carbonic acid, the greater is the quantity given off in the lungs. 



If, instead of nitrogen, hydrogen or marsh gas be mixed with 

 oxygen in due proportion, and breathed, the compound can be 

 respired for an indefinite time. Some gases, however (such as 

 hydrochloric acid, sulphurous acid, ammonia, chlorine, fluorine, and 

 others), cannot be respired, because their action is so irritating to 

 the breathing organs that the entrance to the windpipe becomes 

 involuntarily closed against them. 



Other gases can be respired, but are poisonous if they are 

 respired. Such are sulphuretted hydrogen, phosphuretted hydrogen, 

 nitric oxide, carbonic oxide, and some others. 



If pure oxygen be respired, it removes the carbonic acid from the 

 blood too rapidly, so that the blood thereby ceases to possess a 

 necessary element for effecting the respiratory changes. The result 

 of this is death by what is called apnoea. Death produced by 

 absence of oxygen, is called asphyxia. 



The gaseous interchanges which take place in the lungs, are not 



