402 ON RESPIRATION. 



the amount is subject to great variation under particular circumstances. Thus, 

 during a state of muscular activity it is greatly increased. Mr. Newport has 

 noticed that in Insects the difference is enormous, their respiration being as 

 feeble as that of cold-blooded animals when they are at rest, and more energetic 

 (the quantity of oxygen consumed in proportion to their size being greater) 

 than that of any other animals, when they are in active movement. In Man 

 the difference is not so great, and its exact amount cannot be readily estimated ; 

 but it is unquestionable that an increase does take place. It has been ascer- 

 tained by Dr. Prout, however, that, if the exercise be prolonged so as to occa- 

 sion fatigue, a diminished consumption of oxygen takes place ; he also states 

 that the exhilarating passions increase, whilst the depressing passions (as also 

 the use of alcohol and tea) diminish, the quantity of carbonic acid exhaled. 

 There is little doubt that there is a great diminution, also, during sleep ; this 

 may be partly due to the total cessation of muscular exertion, and partly to the 

 greater retention of the heat which is the consequence of it. For it appears 

 that the amount of carbonic acid produced is greatly influenced by the tem- 

 perature ; in the Guinea-pig, according to Crawford, the quantity exhaled at 

 104 is only half that which is generated at 55.* The final cause or purpose 

 of this connection will be evident, when we consider the subject of Animal 

 Heat. 



536. It has been supposed, until recently, that the azote of the air undergoes 

 no change through Respiration ; but the experiments of Dr. Edwards have 

 shown that, although its quantity may remain nearly the same, there is a con- 

 tinual absorption and a continual exhalation of the gas. If the absorption be 

 the more active, there will be a disappearance of azote from the air ; if exha- 

 lation predominate, the proportion of this gas will be increased. Even in the 

 same animal, there may be a variation in this respect at different periods of 

 the year, and even at different parts of the day. Thus in nearly all the lower 

 animals on which he experimented, there was an augmentation in the quantity 

 of azote during the summer, sometimes equaling-, in the course of the day, the 

 whole bulk of the animal. On the other hand, towards the end of October, 

 he found that a diminution of the nitrogen began to be apparent ; and this, 

 continued until the following spring. 



537. The reaction which takes place between the air and the blood, is easily 

 explained upon physical principles. If the Blood come to the Lungs charged 

 with Carbonic acid, and is exposed in their cells to the influence of atmo- 

 spheric air, which is a mixture of Oxygen and Nitrogen, an endosmose and 

 exosmose of gases will take place, according to certain fixed laws.t The Car- 

 bonic acid of the blood will pass out, to be replaced by Oxygen and Nitrogen ; 

 and the quantity of the former which enters will be much greater than that of 

 the latter, on account of the superior facility with which oxygen passes through 

 porous membranes. If the venous blood also contain Nitrogen as well as car- 

 bonic acid, this also will pass out, to be replaced by the Oxygen of the air. 

 Thus, there will be a continual Exosmose of Carbonic acid and Nitrogen, and 

 a continual Endosmose of Oxygen and Nitrogen ; and the relative quantities 

 of these gases exhaled and absorbed will be subject to continual variation from 



* The experiments of Dr. Malcolm (Edinb. Monthly Journal, Jan., 1843), appear to 

 show that the proportion of carbonic acid exhaled is greatly diminished in typhus fever. 

 According to Dr. Prom's experiments, the average proportion generated in healthy respi- 

 ration, between 11 A. M. and 1 P. M., is about 3-96 per cent, of the whole inspired air. 

 But in some severe cases of fever, the proportion was as little as 1-18 per cent.; but in 

 general it was about 2-50. The proportion did not seem to be much influenced by the 

 number of respirations; being about the same when they were only 20 as when 48 per 

 minute. 



f See Principles of General and Comparative Physiology, 4379. 



