304 Circulation in the Vcrtoifi, &V. [Book lX< 



other amphibious animals is intermediate between that 

 of cold-blooded animals and warm-blooded. 



The heart has two diftinct auricles, without any com- 

 munication : and under thefe, there is the appearance 

 of two ventricles fimilar in lhape to thofe of the latter 

 clafs : but they may be confidered as one cavity ; for 

 the ventricle fends out not only the pulmonary artery, 

 but likewife the aorta j for there is a paffage in the 

 feptum, by which the ventricles communicate freely, 

 and the blood pafies from the left into the right one. 

 From the aorta the blood returns into the right auricle, 

 while that from the pulmonary artery returns to the 

 left auricle, from which it is fent to the left ventricle, 

 &c. fo that only a part of the blood is fent to the lungs, 

 the reft going immediately into the aorta ; hence the 

 -animal is not under the necefiity of breathing fo often 

 as otherwife it would be. 



The ends of the arteries are the beginnings of the 

 veins, which uniting, as the arteries divided, at length 

 form large trunks, which generally correfpond with the 

 trunks of the arteries, from which, by the medium of 

 fmaller branches, they received their contents. 



But though all arteries terminate in veins, yet the 

 minutenefs of their ramifications, before this takes place, 

 is various; while fome tranfnnit the red globules r others 

 exclude them, and tranfmit nothing but ferum. 



A circumftance contributing greatly to-the progrefs 

 of the blood in the veins is their valvular ftru<5ture, 

 fitting them for deriving affiftance from preflure j -and 

 we find accordingly in the limbs, and wherever elfe' 

 any advantage could be obtained from this circumftancey 

 that the veins are furnifhed with valves, while in the 

 cavities of the body, where they are not fo much, preffed 

 by the action of mufcks, this part of their ftrudure is 

 Wanting. 



The 



