152 SCIENCE FROM AN EASY CHAIR 



sent a general average ; there are healthy men whose 

 pulse usually is less than 45 per minute, and there are 

 individuals who, without being invalids, yet have the 

 movement of the heart so liable to increase in rapidity 

 through mental or other excitement, acting by nerves 

 directly on the heart muscle, that the pulse often goes up 

 to 1 20. In the horse and the ox the pulse or heart beat 

 is 36 to 40 a minute ; in the sheep 60 to 80 ; in the dog 

 100 to 120; in the rabbit 150; and in small creatures, 

 like mice and moles, 200, and even more ! I do not know 

 what is the record for the elephant, but as it seems that 

 the larger the mammal the slower the pulse, one would 

 not expect more than 20 to 25 beats a minute in his 

 case. 



It is easy to watch the beating of the heart of a flea 

 or other small insects under the microscope since the 

 skin is sufficiently transparent. It is not usually much 

 more rapid than in man, but in the very transparent little 

 fresh-water shrimps which are called water-fleas (Ento- 

 mostracd) I have seen the heart beating so rapidly that I 

 could not count its rate. The heart in insects and shrimps 

 and their like is remarkable for the fact that whilst it 

 pumps out blood through arteries both in front and behind, 

 it has no actual veins opening into it. All the veins, which 

 in their ancestors entered the heart in a row on each side 

 of it, have united, and their walls broken down, so that 

 the heart lies in a sac full of venous blood from which it 

 draws its fill, when it dilates, through a series of valve- 

 bearing openings on its surface, openings which, in an 

 earlier stage of development, were connected with indi- 

 vidual veins. 



The heart of the Ascidians or sea-squirts, common 

 sac-like marine creatures of most varied form, size, and 

 colour, is perhaps the most extraordinary in the whole 

 animal series. I have often watched it in transparent 



