[ 302 ] [Book IX, 



CHAP. XXIX. 



CIRCULATION OF THE BLOOD. 



Striiflure cf the Heart, and Circulation in cdd-blooded Animals.-^ 

 Circulation in the e uoarm-Llcoded Animals. -dcurfe cf the Blood 

 through the Lungs .-^-Through the reft of the Body. Ramifications 

 cf Arteries. / alvular Structure of Feins.-* Different from ihc 

 Structure of Lymphatics. ' 



TH E ftructure and ufes of the organs concerned 

 in the circulation of die blood have been al- 

 ready coniidered, and it was farther remarked that 

 the heart of man is of a duplex conftruction, in other 

 words, that it has two auricles and two ventricles* 

 With a view to perfpicuity, before we proceed to the 

 circulation in the human body, it will be neceffary to 

 mention the ftructure of the heart in certain animals 

 in which it is more fimple. 



In frogs, ferpent^, and other cold-blooded animals> 

 the heart confifts of only two cavities, an auricle and a 

 ventricle j from the auricle the blood paiTes into the 

 ventricle, from the ventricle it is driven into the 

 arteries, from the arteries it is received into the veins^ 

 and by the veins is again brought back to the 

 auricle. 



This being well underftood, it cannot be difficult 

 to comprehend the courie of the circulation in man, 

 and the warm-blooded animals, in which the only 

 difference is, that the heart being double, or con- 

 fiding of four cavities, the blood performs two circles 

 inftead of one. From the anterior auricle the blood 

 paffes into the anterior ventricle ; froro the anterior 

 ventricle "it is conducted by the pulmonary artery to 



the 



