40 THE HUMAN BODY. 



The Internal Medium. The same close relationship 

 between the living being and its environment, and the 

 same cyclical interchange between the two which we find 

 in the amoeba and the yeast-cell, occur also in even the 

 most complex living beings^ When, however, an animal 

 comes to be composed of many cells, some of which are 

 placed far away from the surface of its body and so from 

 immediate contact with the environment, there arises a new 

 need a necessity for an internal medium or plasma which 

 shall play the same part toward the individual cells as the 

 surrounding air, water, and food to the whole animal. This 

 internal medium kept in movement, and receiving at some 

 regions of the bodily surfaces materials from the exterior, 

 while losing other substances to the exterior at other sur- 

 faces, thus forms a sort of middleman between the in- 

 dividual tissues and the surrounding world, and stands in 

 the same relationship to each of the cells of the Body as 

 the water in which an amoeba lives does to that animal or 

 beer-wort does to a yeast-cell. We find accordingly the 

 Human Body pervaded by a liquid plasma, containing gases 

 and food material in solution, the presence of which is 

 necessary for the maintenance of the life of the tissues. 

 Any great change in this medium will affect injuriously 

 few or many of the groups of cells in the Body, or may even 

 cause their death; just as altering the media in which 

 they live will kill an amoeba or a yeast-cell. 



The Blood. In the Human Body the internal medium is 

 primarily furnished by the blood, which, as every one 

 knows, is a red liquid, very widely distributed over the 

 frame, since it flows from any part when the skin is cut 

 through. There are in fact very few portions of the Body 

 into which the blood is not carried. One of the exceptions 

 is the epidermis, or outer layer of the skin : if a cut bo 

 made through it only, leaving the deeper skin-layers in- 

 tact, no blood will flow from the wound. Hairs and nails 

 also contain no blood. In the interior of the Body the 

 epithelial layers lining free surfaces, such as the inside of 

 the alimentary canal, contain no blood, nor do the hard 

 parts of the teeth, the cartilages, and the refracting media 



