252 FUNCTIONS OF NUTRITION. 



capacity of the blood for oxygen depends on the presence or absence 

 of the red globules. According to Magnus, while the blood contains 

 more than twice as much oxygen as water could hold in solution at. the 

 same temperature, the serum alone has no more solvent power for this 

 gas than pure water ; and on the other hand, defibrinated blood, that 

 is, the serum and globules mingled, dissolves as much oxygen as the 

 fresh blood. Pfliiger found, as the average of six observations on the 

 arterial blood of the dog, that the oxygen in the entire blood was, 

 by volume, 15.6 per cent., while in the serum alone there was only 

 0.2 per cent. According to the same observer, the arterial blood in 

 the carotid contains nearly though not quite all the oxygen which 

 it is capable of holding in solution ; since a specimen of dog's blood 

 drawn directly from the artery contained 18.8 per cent, of oxygen, 

 which was only increased to a little less than 20 per cent, by agitation 

 with atmospheric air. The blood, therefore, either does not become 

 fully saturated with oxygen in passing through the lungs, or else a 

 little of this gas has already passed into some other combination before 

 reaching the carotid arteries. 



The color of the blood depends on the presence or absence of oxygen, 

 not on that of carbonic acid. Yenous blood, shaken up with oxygen 

 or atmospheric air, at once assumes the arterial tint, though its car- 

 bonic acid may remain. According to Pfliiger if defibrinated dog's 

 blood be placed in two flasks, and shaken up, one with pure oxygen, 

 the other with a mixture of oxygen and carbonic acid, both specimens 

 will present the same bright color ; both of them being found on 

 analysis to contain nearly the same quantity of oxygen, while their 

 proportions of carbonic acid are different. If blood be drawn after the 

 animals have been made to breathe pure oxygen, or oxygen and car- 

 bonic acid mingled, it is of the same color in each instance; its 

 percentage of oxygen being the same, while that of carbonic acid is 

 different in the two cases. 



It is the oxygen, therefore, which, on being taken up by the blood- 

 globules, changes their color from dark purple to bright red. It passes 

 off with the arterial blood in this condition, and is then distributed to 

 the capillary circulation. Here, as the blood comes in contact with the 

 tissues, its oxygen in great measure disappears, and its color is again 

 changed from arterial to venous. 



The loss of oxygen in the capillaries of the general circulation, is 

 due to its transfer from the blood-globules to the tissues. Nearly all 

 the tissues exert an absorbent power upon oxygen, when exposed to 

 this gas or to atmospheric air. The experiments of Paul Bert* have 

 shown that the fresh tissues, taken from the body of the recently 

 killed animal and exposed to the air in closed vessels, absorb oxygen 

 with different degrees of intensity, in the following order, namely: 

 muscles, brain, kidneys, spleen, testicle, and pounded bones. Of these 

 the muscles are the most active, absorbing 50 cubic centimetres of 

 * Legons sur la Physiologic comparee de la Respiration. Paris, 1870, p. 46. 



