OF ANIMAL BODIES. 95 



satisfy the question, as to the need which the blood stands 

 in, of being visited by continual accesses of air, is not for 

 us to inquire into ; nor material to our argument : it is suf- 

 ficient to know, that, in the constitution of most animals 

 such a necessity exists, and that the air, by some means or 

 other, must be introduced into a near communication with 

 the blood. The lungs of animals are constructed for this 

 purpose. They consist of blood-vessels and air-vessels ly- 

 ing close to each other ; and wherever there is a branch 

 of the trachea or windpipe, there is a branch accompanying 

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

 middle between the blood-vessels.* The internal surface 

 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 superfices of fifteen feet square. Now 

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

 organization, (and this is the part of the subject with which 

 we 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 it 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 under- 

 gone the action, whatever it be, of that viscus, it is 

 brought back by a large vein once more to the heart, m 

 order, when thus concocted and prepared, to be from 

 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 ne- 

 cessary ; and four are accordingly provided : two, call- 

 ed ventricles, which send out the blood, viz. one into the 

 lungs, in the first instance ; the other into the mass, after 

 it has returned from the lungs : two others also, called 

 auricles, which receive the blood from the veins; viz. one, 

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

 same blood comes a second time after its circulation 

 through the lungs. So that there are two receiving cavi- 

 ties, and two forcing cavities. The structure of the heart 

 has reference to the lungs, for without the lungs one of 

 each would have been sufficient. The translation of the 



^Keill's Anat. p. 121. 



