340 



THE AMERICAN NATURALIST 



[Vol. XLIX 



within wide ranges of barometric pressure. There is 

 some difference in the processes of absorption at high and 

 low barometric pressures (Haldane), but the end result is 

 essentially the same. No more oxygen is taken up by the 

 hemoglobin at pressures of two or three atmospheres 

 than at a pressure of half an atmosphere although the 

 amount of oxygen dissolved in the fluid portion of the 

 blood may be greater at the higher pressures. This dif- 

 ference is insignificant compared with the total oxygen in 

 the blood. In the lower forms, manganese in the Echino- 

 derms, and copper in certain Crustaceae take the place of 

 the iron in the respiratory pigment. The general condi- 

 tions under which the oxygen is carried in the blood are, 

 however, essentially the same in the various forms. 



4. The Concentration of Sugar in the Blood 

 It has been stated since the time of Claude Bernard 

 that the blood in the portal vein at the height of digestion 

 contains more sugar in unit volume than the blood in the 

 hepatic vein. The liver of the homoiothermal animals 

 converts sugar to glycogen and stores it up in that form. 

 Further extensive storage of glycogen occurs in the 

 muscles. 



While, under ordinary circumstances, there is good 

 reason to believe that the concentration of sugar in the 

 blood of a given animal is fairly constant for a given set 

 of conditions, it is known that the amounts of sugar in the 

 blood may vary under other conditions. Excessive 

 amounts of sugar in the blood are eliminated by the kid- 

 neys. If the concentration of sugar falls, the transfor- 

 mation of glycogen to sugar makes up the deficiency. The 

 concentration of sugar in the blood is not necessarily the 

 same for any two species of animals under essentially the 

 same conditions. Nor does the concentration of the sugar 

 in the blood remain constant in any one individual, cat, 

 dog, or human, under all conditions. 32 



32 Cannon, American Journal of Physiology, 1M4, XXXIII, p. 257; 

 Scott, Ibid., 1914, XXXIV, p. 271; Shaffer, Journal of Biological Chem- 

 istry, 1914, XIX, p. 297. 



