GENERAL CONSIDERATIONS. 355 



the utmost regularity. It must be remembered that the es- 

 sential processes of respiration take place in all the tissues 

 and organs of the system, and not in the lungs. Kespi ration 

 is a process similar to what are known as the processes of 

 nutrition ; and although it is much more active and uniform, 

 as far as its products are concerned, than the ordinary nutri- 

 tive acts, it is inseparably connected with, and strictly a part 

 of the general process. As in the nutrition of the substance 

 of tissues, certain principles of the blood, fibrin and albu- 

 men, for example, united with inorganic principles, are used 

 up, transformed into the tissue itself, finally changed into 

 excrementious products, such as urea or cholesterine, and dis- 

 charged from the body, so the oxygen of the blood is appro- 

 priated, and carbonic acid, which is an excrementitious prod- 

 uct, produced whenever tissues are worn out and regener- 

 ated. There is a necessary and inseparable connection be- 

 tween all these processes ; and they must be considered, not 

 as distinct functions, but as different parts of the one great 

 function of nutrition. As we are as yet unable to follow out 

 all the changes which take place between the appropriation 

 of nutritive materials from the blood, and the production of 

 effete or excrementitious substances, it is impossible to say 

 precisely how the oxygen is used by the tissues, and how the 

 carbonic acid is produced. We only know that more or less 

 oxygen is necessary to the nutrition of all tissues, in all ani- 

 mals, high or low in the scale, and that they produce a cer- 

 tain quantity of carbonic acid. The fact that oxygen is con- 

 sumed with much greater rapidity than any other nutritive 

 principle, and that the production of carbonic acid is corre- 

 spondingly active, as compared with other effete products, 

 points pretty conclusively to a connection between the ab- 

 sorption of the one principle and the production of the other. 

 In asphyxia, indeed, it is difficult to say which destroys 

 life, the absence of oxygen or the accumulation of carbonic 

 acid. 



In some of the lowest of the inferior animals, there is 



