488 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1910. 



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. Accordingly they have relatively 

 very large hearts — 1 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 25 times as much as that of a plaice of the same weight, and 

 is about equal to that of a salmon 15 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. 



Frequenc}^ 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 (Telestes and Bar- 

 bus) 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 10 and 70 per 

 minute at room temperature, and no elevation of temperature raises 

 it to beyond 125 per minute (1), 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 vol- 

 ume 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, amphib- 

 ians, and reptiles) we know very little as to the special demands 

 made upon the heart. It has certainly more work to do in amphib- 

 ians 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 the circulation in fish (1). The 

 relative heart size is accordingly greater in amphibians and reptiles. 

 In the frog {R. tem'porarid) and in a crocodile the heart was found 

 to be about 0.4 per cent of the body weight and to be nearly 0.8 per 

 cent in the common snake (7). But we do not as yet know what the 

 tissues take most from the blood in these lower vertebrates; we only 

 know for a certain number of species of fish and amphibians and for 

 the crocodile (6) that they take very little oxygen and that the rate 



