SIGNIFICANCE OF PULSE EATE BUCHANAN. 499 



do not here concern lis; the white below is the acid and the black 

 the mercury. The record reads from right to left. It wall be seen 

 that the acid moved toward the mercury at regidar intervals. These 

 can be counted; in this particular photograph 30| of them occur in 

 the two seconds, indicating that the heart was beating at the rate of 

 nine hundred and fifteen times per minute. 



Until this method was introduced the frequencies of beat in small 

 warm-blooded animals were not actually known. Their order had, 

 however, already been infeiTed by Dr. Haldane from the known 

 quick rate of consumption of oxygen. The method he introduced 

 some 14 years ago of detecting the presence of carbon monoxide in 

 mines, which has been the means of averting many disasters, depends 

 essentially upon the fact that the more rapid the circulation is 

 through the lungs, the more quickly is an animal affected by poison- 

 ous gases absorbed from the atmosphere and the more quickly does 

 it recover in air free from such gases. Since carbon-monoxide, which 

 is far more dangerous to life than any of the other gases which are 

 formed when explosions or fires occur in mines, neither affects the 

 sense organs nor produces pain, miners may remain unaware of its 

 existence and so do nothing to avoid it, until they suddenly succumb. 

 Had they only with them a mouse or a small bird in a cage, forming 

 as much a part of their equipment as a safety lamp, they would have 

 sufficient time to escape from a place which is dangerous, by leaving 

 as soon as the animal showed symptoms, long before they themselves 

 had absorbed a sufficient quantity to be incapacitated. If they are 

 quick, the animal wnll live to aid them in finding a safe place of 

 retreat. As it takes 14 to 15 times as long when at rest and 7 to 8 

 times as long when at work, for a man to be disabled as for a mouse, 

 the miner, even if working, would have one or two hours for escape 

 W'ith such percentages of carbon-monoxide in the air as usually occur 

 in mines (14). 



The frequency of beat, as we have seen, has not become adapted 

 by itself to regidate the supply of oxygen to the demands of the 

 different warm-blooded animals, but other factors also play their 

 part. We have shown that of these the principal one is the volume 

 of blood expelled per beat. ^Ye have now to inquire what signifi- 

 cance is to be attached to the fact that now the one and now the other 

 of the two main re^dating factors plays the more important part. 



Parrot's observations on the relative heart weights of over 50 dif- 

 ferent species of birds and those others of birds and maunnals re- 

 ferred to in our tables show that the relatively large heart is found 

 in the more active animals. This is so not only in warm-blooded 

 animals, but also, as we have already noticed, in fish, flatfish having 

 a relative heart weight less than half that of more active fish. It is 

 probably also the case in amphibians and reptiles, although we have 



