No. 582] SIGNIFICANCE OF INTERNAL CONDITIONS 337 



of them as hibernate, is significantly higher than in the 

 poikilothermal animals. The precise significance of this 

 higher pressure in the homoiothermal animals is un- 

 known. It has been suggested that a certain pressure is 

 necessary to overcome the friction of the blood against 

 the walls of the blood vessels. It would appear that fully 

 as much friction might he encountered in the vessels of a 

 turtle weighing thirty or more kilograms as in the vessels 

 of a guinea-pig weighing less than one kilogram. Yet 

 the guinea-pig has the higher blood pressure. Nor does 

 the difference in blood pressure appear wholly due to 

 mere differences in viscosity of the blood of the two 

 forms. 



The general stages, from the point of view of function, 

 in the phylogenetic development of the vascular system 

 have been indicated elsewhere. 28 



In connection with the question of the role of a more 

 or less constant blood pressure in the animal economy, 

 we may mention the experiments of Legallois, Schiff and 

 Goltz. 29 These investigators found that, while the cells 

 of an animal do not die immediately after the blood pres- 

 sure reaches a low level, the life of the cells in such an 

 animal as the frog is not possible for indefinite periods of 

 time, and in rabbits or dogs, death is a matter of hours. 

 AVe may, perhaps, imagine that the low blood pressure 

 may give rise to changes in the chemical systems in the 

 cell that are incompatible with indefinite existence. 



3. The Respiratory Mechanism 

 The respiratory movements in mammals, and probably 

 also in birds, are, as Haldane and Lorraine Smith have 

 shown, kept up at such a rate as will maintain a constant 

 tension of carbon dioxide and oxygen in the alveoli of the 

 lungs, and presumably in the blood leaving the lungs. 

 The oxygen and carbon dioxide content of arterial blood 

 may be supposed to be fairly constant in any one indi- 



