RELATIONS TO NUTRITION. 475 



appropriation and change of materials furnished by the blood. 

 As far as the respiration of these parts is concerned, we can 

 only say, that in this process, carbonic acid is produced and 

 oxygen is consumed. These facts show that respiration is 

 essentially a phenomenon of nutrition, possessing a degree 

 of complexity equal to that of the other nutritive processes. 

 It must be acknowledged that thus far its cause and intimate 

 nature have eluded investigation. In respiration by the tis- 

 sues, no one has yet been able to give the cause of the ab- 

 sorption of oxygen or the exhalation of carbonic acid ; or to 

 demonstrate the condition in which oxygen exists when once 

 appropriated, or the particular changes which take place, 

 and the principles which are lost, in the formation of carbonic 

 acid. 



The views of physiologists with regard to the essential 

 processes of respiration, before the time of Lavoisier, have 

 barely an historical interest at the present day ; except the 

 remarkable idea of Mayow, which comprehended nearly the 

 whole process, and which was unnoticed for about a hundred 

 years. 1 It is not our object to dwell upon the various theo- 

 ries which have been proposed from time to time, or even 

 to fully discuss, in this connection, the combustion theory as 

 proposed by Lavoisier, and modified by Liebig and others. 

 Though this theory is nominally received by many physiolo- 

 gists of the present day, it will be found that most of them, 

 in accordance with the facts which have since been developed, 

 really regard respiration as connected with nutrition. They 

 only differ from those who reject the combustion theory, in 

 their definition of the term combustion. Lavoisier regarded 

 respiration as a slow combustion of carbon and hydrogen ; 

 and if every rapid or slow combination of oxygen with any 

 other body be considered a combustion, this view is abso- 

 lutely correct, and was proven when it was shown that oxygen 

 united with any of the tissues. Longet says that since the 

 time of Lavoisier it is agreed to give the above signification 

 1 See page 411. 



