LYMPH CIRCULATION 43 



for bringing away the waste. This is called lymph. 

 Nature always supplies a surplus of this liquid food ; 

 and yet not being wasteful, she carries this lymph 

 back and again pours it into the blood. To get this 

 back there is in the higher animals a very large system 

 of vessels, along which the fluid is pressed by the 

 action of small vessels — as a sponge absorbs water. 

 But in the amphibians and some other low forms, 

 this fluid may move in large spaces between muscles 

 or in long sacs (sinuses) under the skin or other mem- 

 branes ; and since these easily expand under pressure 

 the spongelike action (capillarity) does not move the 

 fluid properly. So Nature has made these spaces and 

 sacs (sinuses) to pulsate and thus send their contents 

 onward. They are therefore called " lymph-hearts." 

 Now if we look closely at a frog, we can see 

 places on its sides " beat " as if he had the " heaves " 

 or " thumps," to use a horseman's words. There may 

 be one or more fluttering places on each side, and they 

 do not all throb at once or with any regularity with 

 each other. There is one on each side of the tail. 

 If you did not know how a frog breathed you might 

 think that this pulsing was his way of getting his 

 breath. In the amphibians these great lymph-canals 

 often surround the blood-vessels ; but this is not the 

 case in man. There are two of these lymph-hearts in 

 some birds also, as the goose, at the root of the tail. 

 Unlike other hearts they degenerate as the creature 

 gets higher. 



