CHAPTER VI 

 EPITHELIUM 



EPITHELIUM is a tissue whose cells line all outer and inner surfaces of 

 the animal body. In this position these cells are called upon to make all 

 transfers of material from the outer world into the tissues of the body and 

 from the body to the exterior. Epithelium must also act as an agent for 

 the transfer of sensation and for the mechanical protection of the organs 

 from the rubs and knocks of the surrounding media and objects. 



Its position, with its cells touching the surface of the body, is its chief 

 distinction, and it is only in consequence of this position and the duties 

 that accompany it that the cells are modified into the great variety of 

 structure that we find among them individually and collectively. 



These cells have a strong polarity, a differentiation into an inner and 

 an outer surface or end. These ends differ strongly, according to the 

 work that they have to do, the outer end usually being more highly differ- 

 entiated than the inner. This difference accords with, first, the physio- 

 logical fact that the outer ends of the cells are subject to the greater 

 variety of conditions which determine their differentiation; and secondly, 

 materials for the elaboration of excretion and secretion products are 

 taken from the intercellular fluid or blood by the inner ends of the cells. 

 These substances are all much alike. By a series of processes this mate- 

 rial is finally delivered at the outer ends of the cells in the form of excre- 

 tion or secretion particles. The variety of these elaborated bodies gives 

 a greater variation to the outer than to the inner ends of the epithelial 

 cells. 



The sides are arranged, with rare exceptions, for but one purpose, 

 that of fitting together with those of the surrounding epithelial cells. 

 The commonest shape of cell, in consequence, is that with six sides, ap- 

 proximately conforming to the mathematical requirements of the occasion. 

 The number of sides is variable in different epithelia or even in the 

 same one, and owing to irregularities of arrangement may be more or 

 less than six. Sometimes there are many sides in some cell that is placed 

 in a position where it is surrounded by a large number of others. 



The cells of an epithelium are cemented together by the sides more or 

 less firmly, but at the point or line where they touch each other, at or near 



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