CIRCULATION AND THE HEART 127 



is found in the fishes and amphibians, pulsates, and 

 the one above the heart, situated in the artery, is like- 

 wise inclined to be active. The ventricle also tends 

 to be divided so as to give a good circulation. 



While the lizards, higher up, may have better 

 hearts in some respects, they may have often poorer 

 lungs, and hence the matter is about balanced. Some 

 lizards show quite a tendency to have a partition in 

 the ventricle, and the crocodilians succeed in doing 

 so, and hence their hearts are four-chambered. 



"We recall in the low amphibians and the fishes 

 that there are three or four branches running up on 

 each side from the great tube which starts forward 

 from the heart where the bulb throbs — one to each 

 gill-arch. The frogs had three. Normally the Rep- 

 tiles retain only one of these on each side (a few keep 

 more), which seems to be a very symmetrical (well- 

 balanced) arrangement. To show the onward step, 

 we might mention that the mammals and birds keep 

 only one of these and that on one side, the mammals 

 retaining the left and the birds the right. This is the 

 case also in the monitor-lizards and the chameleon, an 

 onward hint, evidently, in this 'cold-blooded class. 

 In the livahlesB Am^MshcBnidce also there is only one. 

 But in the crocodilians, although the heart is perfect, 

 there remains, two of these forward _ tubes ; and after 

 leaving the heart, they cross each other, and there is 

 at the crossing an opening from one to the other. 

 The rule is, that in other reptiles the blood comes 

 from the system (through an auricle) into the single 

 pumping-chamber, from which three tubes run— the 



