78 POSITITION OF THE HEART, 



trunks are united together. .But still, I have no idea 

 how the heart is formed so as to be made a moving pow- 

 er, nor whence it derives its power. 



Dr. B. The clearest idea that we can have of the 

 nature and purposes of the heart, is by considering it as 

 it exists in its simplest condition in the inferior aniftials^ 

 merely as a hollow muscle placed somewhere in the cir- 

 cle, through which the blood passes and receives from its 

 impulse an increased rapidity of motion. 



Emily. I had forgotten to ask whether it was in the 

 trunks of the veins or arteries that the heart is placed. 



Dr. B. A question well asked, for the position of the 

 heart in respect to the other parts of the circulating sys- 

 tem would not be a matter of indifference. The truth is 

 that the heart considered merely as a moving power, may 

 be placed either where the venous, or the arterial trunks 

 join. In the former case, the blood will be driven with 

 increased impetus towards the respiratory organs, but its 

 motion in the arteries will be unaffected by it. In the 

 latter case, the blood will be propelled with accelerated 

 force towards every part of the body, while its progress 

 in the veins remains unaffected. Instances of the heart 

 in each of these two different positions may be found in, 

 the inferior animals in fishes, of the first position ; in some 

 of the shell fish, of the last. Now there is nothing neces- 

 sarily to prevent there being a heart in each of these 

 situations, and thereby the progress of the blood be ac- 

 celerated in both directions. Such is actually the case in 

 some of the molluscous tribes, as the cuttlefish and nau- 

 tilus. Here is a diagram illustrating this form of the 

 circulation. If now we consider these 

 two hearts brought together and join- 

 ed by their sides, their cavities still re- 

 maining distinct, we shall have the 

 heart as it exists in man, quadrupeds, 

 birds and some reptiles. It is there- 

 fore far from being a single heart, but 

 is strictlv a double heart two single hearts united together . 



