26-i THE CIRCULATION. 



CHAPTER XIV. 



THE CIRCULATION. 



THE blood may be regarded as a nutritious fluid, holding in 

 solution all the ingredients necessary for the formation of the 

 tissues. In some animals and vegetables, of the lowest organization, 

 such as infusoria, polypes, alga3, and the like, neither blood nor 

 circulation is required ; since all parts of the body, having a similar 

 structure, absorb nourishment equally from the surrounding media, 

 and carry on nearly or quite the same chemical processes of growth 

 and assimilation. In the higher animals and vegetables, however, 

 as well as in the human subject, the case is different. In them, the 

 structure of the body is compound. Different organs, with widely 

 different functions, are situated in different parts of the frame ; and 

 each of these functions is more or less essential to the continued 

 existence of the whole. In the intestine, for example, the process 

 of digestion takes place ; and the prepared ingredients of the food 

 are thence absorbed into the bloodvessels, by which they are 

 transported to distant tissues and organs. In the lungs, again, 

 the blood absorbs oxygen which is afterward to be appropriated by 

 the tissues ; and carbonic acid, which was produced in the tissues, 

 is exhaled from the lungs. In the liver, the kidneys, and the skin, 

 other substances again are produced or eliminated, and these local 

 processes are all of them necessary to the preservation of the general 

 organization. The circulating fluid is, therefore, in the higher 

 animals, a means of transportation, by which the substances pro- 

 duced in particular organs are dispersed throughout the body, or 

 by which substances produced generally in the tissues are conveyed 

 to particular organs, in order to be eliminated and expelled. 



The circulatory apparatus consists of four different parts, viz : 

 1st. The heart ; a hollow, muscular organ, which receives the blood 

 at one orifice and drives it out, in successive impulses, at another. 

 2d. The arteries ; a series of branching tubes, which convey the 

 blood from the heart to the different tissues and organs of the body. 



