THE PULSE-RATE IN VERTEBRATE ANIMALS 6i 



about o'04 per cent, of the body-weight (2). In birds, on the 

 other hand, the heart has a very large amount of work to do, 

 especially in the birds of passage and those that sing. Accord- 

 ingly ithey have relatively very large hearts — i to 2 per cent, 

 of the body-weight as a rule (3) and sometimes, as in the 

 thrush and golden oriole, as much as about 2'6 per cent. The 

 size of the heart has thus no fixed relation to the size of the 

 animal to which it belongs. The heart of a pigeon, e.g., weighs 

 twenty-five times as much as that of a plaice of the same 

 weight, and is about equal to that of a salmon fifteen times 

 as heavy as the pigeon. A thrush, and a guinea-pig of six or 

 seven times its weight, have hearts of about equal size. 



Frequency of beat, if it be in any way determined by the 

 absolute size of the heart, is certainly no direct function of it. 

 It is true that we have reason to believe, as we shall presently 

 see, that the pulse-rate in the thrush is not very different from 

 what it is known to be in the guinea-pig, but it would also not 

 be very different from what it is in the rabbit, which has a heart 

 of over twice the size. We know very little about what the 

 frequency is in different fish and for those in which it has been 

 accurately determined {Telcstcs and Barbiis) the size of the 

 heart has not been ascertained, though, assuming the relative 

 size to be the same as in the other round fish, we should expect 

 it to be no larger than that of a canary. In both these fish the 

 frequency varies, in different individuals, between 40 and 70 per 

 minute at room temperature, and no elevation of temperature 

 raises it to beyond 125 per minute (i), whereas the heart of a 

 canary may (7) beat with a frequency of 1,000 per minute. 



If the animal made some demand on the heart for a definite 

 volume of blood in unit time, frequency of beat might be 

 expected to bear some relation to the relative size of the heart. 

 Only it would be difficult to discover such a relation unless 

 in a group of animals having the same circulatory arrangements 

 some required a quicker and others a slower circulation for 

 some assignable reason. 



For the lower groups of craniate vertebrates (Fish, Dipnoi, 

 Amphibians and Reptiles) we know very little as to the special 

 demands made upon the heart. It has certainly more work to do 

 in amphibians and reptiles than in fish, having to drive the 

 blood all round the body without the help of the respiratory 

 movements which seem to play so large a part in maintaining 



