TEE INTERNAL MEDIUM. 41 



p03ed of many cells, some of which are placed far away from 

 the surface of its body and from immediate contact with 

 the environment, there arises a new need a necessity for an 

 internal medium o\* 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 sur- 

 faces materials from the exterior, while losing substances to 

 the exterior at the same or other surfaces, forms a sort of 

 middleman between the individual tissues and- the surround- 

 ing 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 in- 

 juriously 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 Uood, 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 be made through it only, 

 leaving the deeper skin-layers intact, 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 of the eye (see Chap. XXXII), but these 

 interior parts are moistened with liquid of some kind, and 

 unlike the epidermis are protected from rapid evaporation. 

 All these bloodless parts together form a group of non-vas- 

 cular tissues; they alone excepted, a wound of any part of 

 the Body will cause bleeding. 



In many of the lower animals there is no need that the 

 liquid representing their blood should be renewed very rapidly 

 in different parts. Their cells live slowly, and so require but 

 little food and produce but little waste. In a sea-anemone, 



