128 STORY OP THE REPTILES 



two already noted as going forward into the body 

 and the one running to the lungs. This latter soon 

 forks — one branch going to each lung. Usually — 

 especially in tortoise-forms — there is such an arrange- 

 ment of openings and valves that much of this blood 

 from the system is forced into the lungs, and this re- 

 turns in such a way as to be driven out into the sys- 

 tem anew and not back into the lungs again. 



The description of these arrangements here is too 

 technical for our purpose. In the crocodilians, how- 

 ever, jsv^here there are two pumping-chambers, the 

 right has two tubes leaving it — one for the system 

 and one for the lungs. Into the lung-tube the already 

 used blood comes from the body. Here it will be 

 seen that the lungs get much of the blood to aerate, 

 but not all of it, since some goes on into the remain- 

 ing artery which runs mainly to the lower part of the 

 body. When the blood returns from the lungs, red 

 and hotter, it reaches the other pumping-chamber and 

 can not mix here with the colder blood. From this, 

 only one artery runs forward, and it goes largely to 

 the forepart of the body, or sends off many branches 

 that way, the rest of it running to the rear. Now if 

 it were not for the hole, where the two tubes cross 

 outside of the heart, the forepart of the body would 

 have well-aerated blood all the time and the rear part 

 some hot blood, but mostly cool blood — a condition 

 just suited to the position when the body is half out 

 and half in the water. We saw that the frog had 

 something like this; and it is quite probable that 

 by such similar arrangements — as fulness of tubes, 



