36 HISTOLOGY. 



many angled form characteristic for embryonic cells. As 

 development goes on, the cells become constantly more unlike 

 one another i. e., a differentiation sets in. In such a multi- 

 cellular organism the differentiated cells no longer subserve all 

 the life functions, as is the case with unicellular animals. There 

 are cells capable of performing only certain duties. We see 

 here the principle of division of labor. These cells differen- 

 tiated in certain directions, combined to perform certain func- 

 tions, and arranged according to certain laws, form the tissues. 

 By a tissue we understand a complex of cells definitely 

 arranged, differentiated in a definite direction, and combined to 

 carry out a definite activity. 



Tissues consist not only of cells, but also of cell products, 

 which we group under the term intercellular substance. This 

 is sometimes a secretion of the cells, and sometimes a product 

 formed by a change in the superficial part of the cell proto- 

 plasm. It is wanting in quite early embryonic tissues and is 

 built up in time by the cells. 



The various tissues unite in manifold ways to form organs 

 i. e., bodies of a definite internal structure, and a constant 

 external form, which serve a special physiological function. 

 Only exceptionally does an organ consist exclusively of one 

 tissue, as, for example, the lens of the eye. Usually many, 

 often all of the tissues are used in tshe building up of the organ, 

 e. ff.j the intestine, the skin, etc. 



The classification of tissues is one of the most difficult 

 problems in histology. It cannot be made on a purely mor- 

 phological basis ; for not only the form, but also the develop- 

 ment and chemical properties of the tissue must be considered. 

 The separation of tissues into groups according to their devel- 

 opment and origin is not satisfactory, since the same tissue may 

 arise in more than one way. The most generally accepted 

 classification of tissues is the following : 



1. Epithelial (and glandular) tissue; 



2. Supporting and interstitial tissue; 



3. Muscular tissue ; 



4. Nerve tissue. 



