246 BIOLOGY. [BOOK n. 



surface of the branchiae, as on the surface of the lungs, there is 

 a continual exchange of gas between the sanguineous liquid and 

 the aquatic medium. The quantity of air dissolved in water is 

 not considerable ; but this air is respired by cold-blooded 

 animals, and it is besides much richer in oxygen than the atmo- 

 spheric air. In effect, the air dissolved in the water is composed 

 of 29-8 of oxygen, 6 6 '2 of azote, and 4 of carbonic acid. Now 

 Humboldt and Provencal observed this air to contain only 2 '3 

 of oxygen, 63*9 of azote, and, in compensation, 33*8 of carbonic 

 acid, after having been respired a certain time by some fishes. 1 



If observation has been able to scrutinize and measure the 

 respiratory phenomena, properly so-called, that is to say, the 

 exchange of gases through the capillary membrane, it has not 

 succeeded so well as regards the nutritive part of the respiration. 

 Nevertheless, interesting facts have been collected, and they 

 permit us already to form a general idea of the office which ab- 

 sorbed oxygen fulfils, and of the succession of chemical trans- 

 formations with which they co-operate. 



The first and most striking phenomenon is the sadden change 

 of colour which the sanguineous liquid undergoes immediately 

 after absorbing the atmospheric oxygen. The blood, which 

 arrived in the capillaries in the state of black or venous blood, 

 becomes rutilant and vermilion, as soon as it has exchanged its 

 carbonic acid for oxygen ; it is then called red or -arterial blood. 

 The change of colour is instantaneous. For example, the blood 

 in the carotid artery of an animal blackens and reddens alter- 

 nately arid suddenly, according as we prevent or permit the 

 access of air to the respiratory apertures. 



Of the oxygen absorbed by the blood, a part remains in a state 

 of solution in the sanguineous plasma, and probably co-operates 

 with the oxydations in the isomeric transformations of the 

 nutriments ; but the larger portion of this oxygen is fixed by 

 the hrematia. These corpuscules absorb many vojlumes of it; 

 they are truly the vehicles of the oxygen. They impregnate 

 1 Mgmoires de la Societt d'Arcucil, t. II. ' 



