^o SCIENCE PROGRESS 



life to ascertain what that volume is for any heart ; we have 

 had therefore to take the only available data which were at all 

 likely to be a measure of it. 



None of the mammals referred to have pulse-rates appreciably 

 higher than those expected by comparison with man. All 

 birds, however, so far as we know, have higher frequencies than 

 would be expected when compared with mammals, e.g. that 

 anticipated for the sparrow would be only 6i8, instead of 800, 

 per minute. 



Carbon-dioxide-output. Pulse-rate. Relative heart-weight. Pulse-rate, 

 sparrow 12-2 _ 1423 0*59 _ 618 



man 06 ~ 70 ' 1-36 ~ 1423 



The haemoglobin-percentage does not appear to have been 

 determined in the blood of birds, but in view of the greater size 

 of the red blood corpuscles of birds as compared with those 

 of mammals we might expect it to be lower. On the other 

 hand, the fact that the consumption of oxygen and both 

 relative heart-weight and pulse-frequency are higher in a bird 

 than in a mammal of the same size {e.g. in the sparrow than 

 in the mouse, in the pigeon than in the guinea-pig) may have 

 some bearing on the fact that birds maintain a higher constant 

 temperature than mammals. 



In this connection it is interesting to note that in the lowest 

 mammals, the Monotremes, and also in the Marsupials, in which 

 a lower body temperature is maintained, the heat produced, as 

 measured by the carbon-dioxide-output per kilo, per hour, is 

 much less than in so-called placental mammals of the same size 

 (10). We know nothing at present about relative heart-size or 

 pulse-rate in these animals. But since the Monotremes, like 

 the small placental mammals, regulate their temperature by the 

 production of more heat when required {i.e. in cold surroundings) 

 instead of by always producing a large amount and getting 

 rid of the excess when necessary, as the larger of the higher 

 mammals do, we should expect the pulse-rate in them to vary 

 a good deal, and inversely, with the external temperature. The 

 Marsupials, utilising also variations in loss of heat although 

 to a less extent than the larger of the placental mammals, 

 seem to regulate their body-temperature extremely well. Of 

 the two Monotremes still living, Ornithorhyncus succeeds in 

 doing so quite as well as some of the placental mammals ; 



