'je SCIENCE PROGRESS 



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 increasing the oxygen-supply by one and 

 a half times at the most ; that of the hare goes up under similar 

 circumstances to 264 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 inter- 

 mission — 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 locomotion." The relative size of 

 the hare's heart, according to Bergmann'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 favoured 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 



