40 



HUMAN PHYSIOLOGY 



mists knew that the bodies of animals were composed 

 of various types of material, the tissues. Thus we 

 say that bone, muscle, skin, the substance of the nervous 

 system, and blood are tissues. In the light of the cell 

 theory we are inclined to think of a tissue as a collec- 

 tion of similar cells. This is not wholly satisfactory, 

 for the character of a given tissue is often more dependent 

 on the nature and amount of the intercellular matter 

 mentioned above than on the peculiarities of the cells. 

 Therefore, it is best to regard a tissue as consisting of 

 associated cells and intercellular accumulations. In 

 some cases, as in the skin, the intercellular deposit is 

 slight; at the other extreme we have cartilage and bone 



FIG. 2. To emphasize the difference between an epithelial tissue 

 (at the left) in which the cells are closely packed and a form of connec- 

 tive tissue (cartilage) consisting mainly of an intercellular deposit. 



where the cells form but a very small fraction of the 

 total mass. 



It was suggested a moment ago that blood is a tissue. 

 This may not be a familiar idea nor one that is universally 

 acceptable. But it will be noted that blood conforms 

 with the terms of our definition. It contains cells of 

 standard kinds with intervening material which in this 

 instance is a liquid. There is more or less intercellular 

 fluid in almost any variety of tissue. 



Four conspicuous orders of tissues may be set apart. 

 They are (1) the epithelial, (2) the connective, (3) the 

 contractile, and (4) the nervous. Those of the first class 

 (epithelial tissues) are the surface tissues, the coverings 



