THE SIGNIFICANCE OF THE PULSE RATE IN VERTE- 

 BRATE ANIMAES.' 



By Florence Buchanan, D. Se. (Lond.), 

 Fellow of Vnivcrsifj/ College. London. 



We should expect the frequency with which a heart beats to be 

 determined by its own properties — by its size, the minute structure 

 of its muscle fibers, the inorganic salts in and outside the fibers, tem- 

 perature, its relation to the nervous system, etc.; and it probably is 

 immediately determined by such things as these. At present, how- 

 ever, we do not know the properties in which the hearts of allied 

 animals, beating with very different frequencies, differ from one an- 

 other, and we are not therefore in a position to point to the imme- 

 diate determining factors. AH we know is that the properties, what- 

 ever they are, which determine frequency have come to be such as to 

 enable the heart to serve the purposes of the animal to which it be- 

 longs. It is proposed in this paper to attempt to ascertain whether 

 we can find out some of the different ways in which the heart serves 

 these purposes, and whether or to what extent alteration in frequency 

 of beat is one. To do this we must first know something about the 

 different purposes for which the heart is required in different ani- 

 mals. 



In the first place, the amount of driving work the heart has to do 

 varies a good deal in the different craniate vertebrates and both with 

 the structure and the habits of the animal. In fish, e. g., it has only 

 to pump the blood as far as the gills, and it has not much to do even 

 in effecting this, as the passive dilatation of the gill capillaries with 

 each inspiratory movement of the buccal cavity helps the blood to get 

 there (1).^ In accordance with this small amount of work, we find 

 the heart to be of relatively small size in fish. Its weight in the 

 common round fish is on the average only 0.09 per cent of the body 

 weight; in the notably inert flatfish it is even less, only about 0.04 



1 From a lecture delivered to the Oxford University Junior Scientific Club in Novem- 

 ter, 1909. Reprinted by permission, with author's additions and corrections, from 

 Science Progress, London, No. 17, July, 1910, pp. 60-81. 



2 These numbers refer to a list of authorities given at the end of the paper. 



487 



