THB HEART. 



is necessary to the simplest form of the circulation a set 

 of vessels to carry the blood to the lungs, and another to 

 distribute it to the body. 



Emily. Thus far, I believe, I understand the matter 

 perfectly well, but I cannot conceive by what means the 

 blood is moved through the vessels ; unless that it be by 

 the action of the vessels themselves, and yet I have al- 

 ways supposed that the heart was connected in some way 

 with the motion of the blood, but you have not spoken 

 of it. 



Dr. B. I have purposely avoided speaking of it, in 

 order that your notions of its relation to the circulation 

 might not be confused by introducing it before you were 

 properly prepared to understand it. Have patience 

 one moment and you shall understand all. In this dia- 

 gram the blood is supposed to be moved soly by the 

 power of the vessels themselves ; no other is present to 

 assist them. Now, in some of the lower orders of ani- 

 mals, this is precisely the manner of the circulation the 

 power of the vessels without any other aid, is snfficient 

 to propel the blood through them. But if we ascend 

 the scale of being to where the organization has increas- 

 ed in complexity and perfection where the vital ener- 

 gies possess more activity and strength where conse- 

 quently, a greater degree of action is required in the ap- 

 paratus which is to support the vital energies and repair 

 their losses, we find an additional force is provided to 

 assist the action of the vessels. Now then, you proba- 

 bly can tell what this force is. 



Emily. I presume it must be the heart, if this organ 

 is concerned in the motion of the blood, for I cannot see 

 what other nse it can have. 



Dr. B. You are right ; and since you have com- 

 prehended the subject so well thus far, you will perhaps 

 perceive at once in what part of the circle the heart, 

 should be placed, in order that it may act with the great- 

 est advantage. 



Emily. Undoubtedly it would be where the two 



