USE OF THE VESSELS. 75 



to perfect health by injecting into its veins the blood of 

 another dog, while that of a man or horse would be fol- 

 lowed by little or no benefit. On the whole, the blood 

 may be considered as the most important of the animal 

 fluids, being the grand source whence the system re- 

 ceives the needful supplies to repair its waste and main- 

 tain its growth. 



In all animals that possess a circulating system, we 

 find at the same time, an apparatus for the function of 

 respiration, and the quantity ', or degree of perfection of 

 one. of these systems is in direct proportion to that of the 

 other. As the blood in its passage through the body 

 loses much of its nurient materials, and acquires others 

 that are no longer fitted for the purposes of life, some 

 provision is evidently required by means of which it may 

 throw off these useless portions, and receive continual 

 renovation. 



Emily. And this provision is furnished by the lungs. 

 I have now a more definite idea of the use of these or- 

 gans and their connection with the circulation, than I 

 ever possessed before. I begin now also, to perceive 

 this beautiful system of harmony of action and mutual 

 dependence, which associates the various organs of the 

 body in one great and perfect whole. 



Dr. B. You now readily see the necessity of the 

 blood's being constantly submitted to the action of the 

 respiratory organs, and as constantly changed by this 

 action. Every portion of blood before being sent to the 

 body must be carried to them, and the essential use of 

 the circulating vessels, considered in their simplest con- 

 dition, is to convey the blood from every part of the sys- 

 tem to the lungs, and thence back to the system again. 

 Here then is a perfect circle of action which is inces- 

 santly going on from the beginning to the end of exis- 

 tence. 



Emily. What a multitude of vessels must be requir- 

 ed thus to carry the blood to and from every and the 



