SIGNIFICANCE OF PULSE RATE BUCHANAN. 501 



knowledge it is difficult to point to any advantage which might accrue 

 to any species of poikilothermic vertebrate from having a particular 

 pulse rate, nor apparently is the variation in different species greater 

 than that in individuals in this respect. 



When frequency came to be correlated with relative heart size 

 for the regulation of the rate of oxygen supply to the heat-forming 

 tissues, the slow pulse would tell as an advantage as well as the large 

 heart in animals having to make great sustained effort; for a slow 

 pulse as compared with a quick one means longer diastoles more than 

 longer systoles, the systole requiring to be very little longer to expel 

 a much larger quantity of blood, since, in contracting, the walls (the 

 surface) of the ventricles decrease with the square, the contents with 

 the cube. The longer the diastole the more time has the heart to 

 recuperate between the beats when the animal is at rest and the 

 greater power has it in time of need of increasing the oxygen supply 

 to the tissues by increasing the frequency of the beat. The pulse rate 

 of the rabbit only goes up to an average of '324 per minute after a 

 few minutes' chasing about or after section of the vagi (11), thus in- 

 creasing the oxygen supply by one and a half times at the most ; that 

 of the hare goes up under similar circumstances to 261 per minute 

 (12), so that if the same amount of blood were expelled in each 

 systole as when the animal was at rest the oxygen supply might be 

 increased as much as four and a half times. MacWilliam, drawing 

 attention to the connection between slow pulse and staying power, 

 remarks (12) with regard to these particular closely allied animals, 

 that " the rabbit is able to run short distances with great rapidity, 

 but not to traverse long distances without intermission — this being 

 no doubt in relation to the fact of their having burrows to flee to; 

 the hare, on the other hand, destitute of such means of protection, 

 has to depend, in the open country, upon its endurance in swift loco- 

 motion." The relative size of the hare's heart, according to Berg- 

 mann's estimations, appears to be nearly three times that of the 

 rabbit's; and of the pulse rates of the two animals at rest that of 

 the rabbit is about three times that of the hare. That staying power 

 rather than wildness itself has led to the larger heart being favored is 

 shown by the fact that there is very little difference in the relative 

 heart weights of the tame and the wild rabbit (9). 



The relatively small heart of animals kept for food, such as the 

 hen, the tame duck, the pig, the ox, and the cow (in which it is 

 the same as in the ox) is, on the other hand, a consequence of the 

 artificial fattening up of these anijnals, thus increasing their body 

 weight, w^hile their hearts, having little to do, do not keep pace, it 

 being possible to supply the oxygen demanded by increasing the 

 frequency of beat. The animals with the smallest hearts would be 

 selected for the purpose in question by man just because of their 



