SIGNIFICANCE OF PULSE KATE— BUCHANAN, 489 



at which this at least is supplied is not likely to cause difficulty. 

 Neither has anything yet been ascertained about differences in pulse 

 rate in different genera of amphibians nor in those of any of the dif- 

 ferent classes of reptiles ; so that we have not the material for decid- 

 ing whether the frequency with which the heart beats has become one 

 of the factors used in natural selection. In the few species of am- 

 phibians and reptiles (6) for which— sporadically — the frequency is 

 known, it seems to be not very different from what it is in fish — i. e., 

 varying (and varying in individuals of the same species) between 

 about 20 and 80 per minute at ordinarj^ room temperature. 



In birds and mammals the case is different. We not only know 

 that the tissues take a great deal of oxygen from the blood, but that 

 those of small animals take much more than those of large ones ; and 

 we can assign a reason. Birds and mammals are able to maintain 

 a nearly constant temperature whatever that of their surroundings 

 may be. They are hommotheTmiie or (the temperature they maintain 

 being usually higher than that of the environment) " warm-blooded " 

 animals; they have in consequence to produce more heat than those 

 animals which maintain no constant temperature — the poikolother- 

 mic or " cold-blooded " animals — and to try to prevent loss of heat. 

 To produce heat the muscles — the chief heat-forming organs of the 

 body — require oxygen, and they take it from the blood according to 

 their need, the need being greatest in those species or individuals in 

 which the loss of heat is greatest. The maximum loss is of course in 

 those animals in which the surface exposed to the environment is 

 largest in proportion to the mass of the animal — i. e., the smaller the 

 animal the more heat must it give off, other things being equal, to a 

 colder environment, and to maintain a constant body temperature the 

 more heat must it produce and the more oxygen must its muscles 

 consume. The heart, therefore, being asked to replenish the supply, 

 must, if it respond, give out the larger relative volume of oxygen- 

 containing blood in unit time the smaller the animal, and it might do 

 so either by expelling a larger amount with each beat or by increas- 

 ing the frequency of the beat. 



But by regulating the volume of blood supjDlied to the muscles in 

 unit time, the heart can only regulate the rate of supply of ox^^gen 

 if the oxygen is present in a constant percentage. This is the case in 

 birds and mammals in which the blood in the systemic circulation 

 leaves the left ventricle of the heart with its hsemoglobin saturated 

 with oxygen. It is not the case in the lower vertebrates, not even in 

 crocodiles ; for although they, like birds and mammals, have the oxy- 

 genated blood completely separated from the rest in the heart, it be- 

 comes mixed with other blood in the dorsal aorta, and may become 

 so cA^en in the conus. In other reptiles facilities for the admixture of 

 the blood coming from the lungs with blood coming from other parts 



