CHAPTER IV 



THE FLUIDS OF THE BODY 



FROM one-fourth to one-third of the whole body is fluid. If 

 the skin be regarded as a watertight bag, three-fourths or 

 rather less of its contents are solid, one-fourth liquid ; and even 

 its apparently solid contents, the tissues, contain much water. 

 Water is an essential constituent of protoplasm. It is also 

 present in cell-juice. The estimate given above does not 

 include the fluid within the cells, but only the fluid with which 

 the cells are bathed. In a general sense this extracellular 

 fluid, excluding blood, is termed lymph. It occupies the 

 spaces of a gauzy " connective tissue," which connects, or 

 separates the terms are equally appropriate muscles, nerves, 

 glands, and other tissues of specialized function. Nowhere, 

 except, in a fashion, in the spleen, does blood come in contact 

 with a cell. The lymph which more or less surrounds them is 

 the bath from which cells receive their food and oxygen, into 

 which they excrete carbonic acid and tissue- waste. The net- 

 work of lymph-spaces is traversed by capillary bloodvessels 

 with walls composed of flattened connective-tissue cells. Such 

 cells are usually spoken of as elements of an " endothelium." 

 As the epithelium covers the surface of the body, so endo- 

 thelium lines its cavities. Endothelial cells are thin scales or 

 tiles with sinuous borders dovetailed one into another. That 

 the tiles which form the walls of capillary vessels are not 

 cemented together in any proper sense is shown by the facility 

 with which white blood-corpuscles, leucocytes, by their amoe- 

 boid movements, push them asunder when making their way 

 from the blood-stream into the tissue-spaces, or vice versa. 

 They offer no more resistance to a leucocyte than a pair of 

 curtains hanging in front of a door offers to a child. Yet so 



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