256 FUNCTIONS OF NUTRITION. 



The production of carbonic acid by the tissues is not, therefore, an 

 immediate result of the absorption of oxygen. The precise mode in 

 which carbonic acid originates in the solid organs is unknown ; but it 

 is probably by some decomposition in which a portion of the carbon 

 and oxygen separate from their previous combinations in this form, 

 while the remaining elements unite to produce other substances of 

 different composition. 



The most palpable phenomena of respiration consist, accordingly, in 

 an interchange of gases between the blood and the lungs. As the blood 

 on its return to the lungs is comparatively poor in oxygen and abun- 

 dant in carbonic acid, it absorbs the former gas from the pulmonary 

 cavity, and discharges the latter with the expired air. These changes, 

 however, are incomplete, both in the air and in the blood. The expired 

 air has never lost the whole of its oxygen, and it contains only about 

 4 per cent, of carbonic acid. On the other hand, venous blood still 

 * contains a moderate percentage of oxygen ; and a certain quantity of 

 carbonic acid is also present in arterial blood. It is only the propor- 

 tion of these gases which is changed in respiration, the carbonic acid 

 of the blood being diminished, and its oxygen increased, during its 

 passage through the lungs. 



The office of the respiratory apparatus is to afford ingress and egress 

 to oxygen and carbonic acid, two substances which enter and leave 

 the body in the gaseous form, but which have no immediate relation 

 with each other, excepting that they are absorbed and exhaled by the 

 same organs. They represent the beginning and the end of a series 

 of internal changes, which are among the most important of those con- 

 nected with the maintenance of life. 



Nature of Respiration. If we regard respiration in its gross results 

 we must consider it as a process of oxidation. The living body absorbs, 

 on the one hand, free oxygen from the atmosphere, and, on the other, 

 takes into the alimentary canal organic substances as ingredients of the 

 food. These organic substances, after performing their office in the 

 system, are discharged from it partly under the form of urea, but 

 mainly as carbonic acid and water. The final products of excretion 

 represent the organic elements of the food, plus the oxygen which has 

 been absorbed ; and they return to the inorganic world in a condition 

 of complete or nearly complete oxidation. These facts are incontestible, 

 and they show plainly the general relations of the incoming and out- 

 going materials of the animal frame. 



But when we endca , or to learn the place and manner of this oxida- 

 tion in the living body, the attempt fails. There is no evidence of such 

 direct action taking place in the circulating fluid, nor in any of the 

 organs or tissues. The food in the alimentary canal, during digestion, 

 undergoes catalytic transformations and solutions, but no oxidation ; 

 and it is absorbed from the intestine with its organic characters unim- 

 paired. In the lungs the process of respiration consists in the absorp- 

 tion of oxygen and exhalation of carbonic acid. These two gases pass 



