318 CIRCULATION OF NUTRITIVE FLUID. 



564. In the warm-blooded division of the Vertebrated series, which 

 includes the classes of Birds and Mammals, we find the whole circu- 

 lation possessed of a greatly-increased energy; but the distinguishing 

 peculiarity of the apparatus in these animals, is that conformation of 

 the heart and vessels, which secures a complete double circulation of 



F} 91 the blood ; that is, which provides for the aeration 



of every particle of the venous blood which has re- 

 turned from the system, before it is again sent into 

 the tissues. The heart may be regarded as con- 

 sisting of two distinct parts, a systemic heart, like 

 that of the Molluscs, forming its left side, and a 

 respiratory heart, like that of Fishes, constituting 

 its right. Each of these parts has a receiving 

 cavity or auricle, and an impelling cavity or ven- 

 tricle. The cavities of the two sides are completely 

 separated from one another, in the adult state at 

 least ; though their walls are united for economy of 

 material. It is obvious that much is saved in this 

 manner; since, as the contractions of the auricles 

 and of the ventricles on the two sides occur simul- 

 taneously, the pressure of blood in the one is partly 

 Diagram of the circulating antagonized by that on the other, wherever it acts 



Apparatus in Mammals and , -P 11 .1. . j. i. ^i mi 



Birds -.-a, the heart, contain- on the wall that is common to both. I his antago- 

 SHvSgTen^ ^To. is not~ complete, however ; since the systemic 



the right auricle -d, the right ventricle contracts with far greater force than the 



ventricle propelling venous o 



biood through e, the puimo- pulmonary ; and the wall between them must be 



nary artery, to /, the capil- - 1 ri *r> . ,1 TCP f 



? of the lungs; g, the left capable of resisting the difference of pressure on 

 its two sides, thus occasioned. The blood which is 

 Jen ventncie A wmcn pro- re turned from the system, in a venous state, through 

 pels it, through 'the aorta, i, the vena cava to the right auricle, and which is 



to the systemic capillaries, -, > .... , . , . i nil. 



j, whence it is collected by poured by it into the right ventricle, is impelled by 

 S thlheMtthroughSfe vena the latter through the capillaries of the lungs, 

 caTa > b - where it undergoes aeration. Returning thence, in 



an arterialized state, it is conveyed into the left auricle, and thence 

 flows into the left ventricle ; by which it is propelled through the great 

 systemic artery or aorta, and through its ramifications to the general 

 system. 



565. The greater part of the blood, which has been rendered venous 

 by passing through the systemic capillaries, is collected by the systemic 

 veins, and is returned directly to the heart through the vena cava. But 

 a portion is still employed for the distinct circulation, which is destined 

 to supply the materials for the secreting action of the liver. The 

 blood that has traversed the capillaries of the walls of the alimentary 

 canal, and of the other viscera concerned in digestion, is collected again 

 by the converging veins into a large venous trunk, the vena portce, by 

 which it is distributed through the liver. This vessel, although formed 

 by the convergence of veins, and conveying venous blood, has really 

 the character of an artery in an equal degree ; for it subdivides and 

 ramifies after its entrance into the liver, so as to form a network of 

 capillaries, from which the blood is again collected, and thence trans- 



