86 RESPIRATION. 



which the function of respiration is performed The influence 

 which the air exerts, is always upon the circulating fluid or 

 blood. It produces some change in it, or imparts some prin- 

 ciple to it, which renders it fit to be distributed to the body 

 for its nourishment. In all the animals which have red blood, 

 viz. the Mammalia, Birds, Reptiles, and Fishes, this change 

 consists, so far as can be observed, in imparting to the dark- 

 red or venous blood, which is sent to the lungs by the heart, 

 a bright red or vermilion color. In this state, it is returned 

 to the heart, and thence distributed throughout the body by 

 the arteries. 



' In the Mammalia, the air is alternately drawn into the 

 lungs, and expelled from them, by the action of the diaphragm 

 and muscles of the ribs. This is called the inspiration and 

 expiration of the air, and is constantly going on in order to 

 produce the requisite change upon the blood, which is con- 

 tiually passing through the lungs. In the greater part of the 

 animals of this class, if this process be stopped but for a few 

 moments, death is the inevitable consequence ; but in some 

 species it may be suspended for, a longer period. This is the 

 case with the seal and the whale. Even men may acquire by 

 habit the power of existing a considerable time without breath- 

 ing, as is the case with the fishermen who dive for pearls ; * 

 but many of the stories which have been related with regard 

 to this subject, are probably destitute of foundation. 



' There are many other kinds of air or gas, which may be 

 taken into the lungs, beside the atmospheric ; but no other 

 which will support life. Even pure oxygen itself, and another 

 gas which contains oxygen, although they will support life 

 longer than any other kinds of air, will yet finally prove fatal. 

 It is only when oxygen is combined with azote in the propor- 

 tion above mentioned, that it is adequate to the continual 

 support of life.t The quantity of air ordinarily contained in 



* The pearl-fishers have been said to remain half an hour or more under water. 

 The accounts, however, which state so extraordinary a fact as this, must he looked 

 upon with great doubt. It seems impossible from what we know on the subject, 

 that any human being could exist and remain capable of action under water, more 

 than a few minutes.' 



f 'Water destroys the life of animals merely by preventing the admission of air, 

 it does not itself eriter the lungs, or at most only in a very small quantity. There 

 are some gases which operate in the same way. The windpipe is spasmodically 

 dosed against them, and they dq not enter the lungs ; such are carbonic acid gas, 

 ammoniacal gas, chlorine or oxymuriatic gas, &c., when unmixed. Other gases are 

 inspired with sufficient ease, but produce death, either merely for the want of oxy- 

 gen, as hydrogen and pure azote; or, in a certain sense, by poisoning the blood or 

 destroying its vital properties, as carburetted and sulphuretted hydrogen, and car- 

 bonic oxide. Oxygen alone, as has been remarked above, and nitrous oxide, which 



