VESSELS OF ANIMALS. 109 



iiiunication with the blood. The lungs of animals are con- 

 structed for this purpose. They consist of bloodvessels and 

 air-vessels, lying close to each other ; and whenever there is 

 a branch of the trachea or windpipe, there is a branch acconi^ 

 panying it of the vein and artery, and the air-vessel is always 

 in the middle between the bloodvessels.'* The internal sur- 

 face of these vessels, upon which the application of the air to 

 the blood depends, would, if collected and expanded, be, in a 

 man, equal to a superficies of fifteen feet square. Now, in 

 order to give the blood in its course the benefit of this organ 

 ization — and this is the part of the subject with which wc 

 are chiefly concerned — the following operation takes place. 

 As soon as the blood is received by the heart from the veins 

 of the body, and before that is sent out again into its arteries, 

 it is carried, by the force of the contraction of the heart, and 

 by means of a separate and supplementary artery, to the 

 lungs, and made to enter the vessels of the lungs; from 

 which, after it has undergone the action, whatever it be, of 

 that viscus, it is brought back by a large vein once more to 

 the heart, in order, when thus concocted and prepared, to be 

 thence distributed anew into the system. This assigns to 

 the heart a double office. The pulmonary circulation is a 

 system within a system ; and one action of the heart is the 

 origin of both. 



For this complicated function four cavities become neces- 

 sary, and four are accordingly provided : two called ventri- 

 cles, which send out the blood — namely, one into the lungs, 

 in the first instance ; the other into the mass, after it ha? 

 returned from the lungs : two others also, called auricles, 

 \vh\:h. receive the blood from the veins — namely, one, as it 

 comes immediately from the body ; the other, as the same 

 blood comes a second time, after its circulation through the 

 hmgs. So that there are two receiving cavities, and two 

 forcing cavities. The structure of the heart has reference to 

 the lung-, • for without the lungs, one of each would have 

 * Keill's Anatomy, p. 121. 



