RESPIRATION. 257 



each other in the pulmonary cavities, on their way to and from the 

 blood ; neither of them being produced or consumed in these organs. 



In the blood, the plasma consists mainly of organic substances in 

 solution, and oxygen is abundant in the globules in a state of loose 

 combination. But the union of the two certainly does not take place 

 in the blood. Oxygen disappears from it in the capillary circulation, 

 and is replaced by carbonic acid derived from the tissues. According 

 to the view now generally accepted, the functions performed by the 

 blood are rather physical than chemical in their nature. It is a vehicle 

 of transportation for nutritious matters from the alimentary canal to 

 various organs, and for oxygen and carbonic acid between the tissues 

 and the lungs. It collects or disseminates substances which have 

 already been prepared in other parts, and, as a general rule, convey- 

 them unchanged to their destinations. Even a substance like pyrogallic 

 acid, so readily oxidizable in an alkaline solution that it is employed 

 for the quantitative determination of oxygen in the air, when intro- 

 duced into the animal system passes through it without alteration, 

 and reappears 'in the urine.* There is no evidence that the blood 

 exerts anywhere a direct oxidizing action. 



Finally, in the substance of the tissues and organs, it is evident that 

 the carbonic acid which they produce is not the immediate result of the 

 absorption of oxygen. Its continued exhalation in an atmosphere of ni- 

 trogen, or of other indifferent gases, shows that it originates, in all prob- 

 ability, by a separation of its elements from other previously existing 

 forms of combination. Furthermore, the alteration of the organic 

 ingredients, so far as we can follow them in the living body, consists 

 largely of hydrations and dehydrations under the influence of the ani- 

 mal ferments. Glucose, when absorbed from the alimentary canal, is 

 reduced, by dehydration in the liver, to the form of glycogen, which, 

 in turn, is again converted by hydration into soluble glucose. The 

 formation of glycocholic from taurocholic acid (p. 109) is a dehydration 

 with elimination of sulphur, while biliverdine is produced from biliru- 

 bine (p. 101) by hydration with oxidation. On the other hand, the 

 derivation of urobiline from bilirubine (p. 102) can only be accomplished 

 by a reduction process ; and the formation of fat in the body from car- 

 bohydrates (p. 63) is undoubtedly accompanied by the elimination of 

 carbonic acid and the production of other substances at the same time. 



From all these facts it appears that the transformation of tissue in 

 the body is not a simple act of combustion, regulated by the supply of 

 oxygen to the lungs. It is one in which the tissues appropriate the 

 oxygen conveyed to them by the blood, to form intermediate com- 

 pounds, and in which they finally eliminate carbonic ~>ncid as the most 

 abundant product of their retrograde metamorphosis. 



* Gorup-Besanez. Lehrbuch der Physiologischen Chemie. Braunschweig, 1878, 

 p. 599. Ewald. Die Lehre von der Verdauung. Berlin, 1879, p. 5. 



K 



