Cells and Tissues. 



resemble wandering cells in their isolation, but differ from them in 

 that they have not the power of independent amoeboid movement ; 

 they are just carried along passively by the blood. 



Most of the cells of the animal body are, however, fixed, they 

 are immovably united to one another and, as they cannot protriide 

 pseudopodia, are fairly constant in shape. These fixed cells are 

 variously modified in the adult: they are specialised, in 

 correlation with differentiation of function. Usually, cells which are 

 modified in the same or in similar ways, are arranged in groups, and 

 such groups are called tissues. Four principal kinds of tissue 

 may be recognised : epithelial, skeletal, muscular, and 

 nervous. 



1. The name epithelium is used to designate those tissues 

 which form a thicker or thinner covering to the outer or inner surface 

 of the body, and which 



consist simply of a num- ' ^ -S 



ber of closely apposed 

 cells. Epithelial cells 

 generally consist of pro- 

 toplasm, in which there 

 may be excretions such 

 as pigment granules, 

 oil globules, etc. : they 

 vary in shape ; some are 

 squamous, the height 

 less than the breadth ; 



others columnar, the height greater than the breadth ; or the breadth 

 and the length may be approximately the same. 



The epithelial cells are joined together by small quantities o£ intercellular 

 cement-substance: often, too, there ai-e delicate strands of protoplasm 

 passing from cell to cell. 



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m 



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Fig. 4. A Simple squamous epithelium, surface view ; 

 B the same in section. Section of simple columnar 

 epithelium. — After Gregenbaur. 



Kg. 5. A Stratified squamous epithelium. B Stratified columnar epithelium. — 



After Gegenbaur. 



Bpithelia may be simple or stratified. Simple epithelium 

 consists of a single layer of closely adherent cells, which may either 

 be fiattened (simple squamous epithelium) or cylindrical (columnar 

 epithelium), or about equal in height and breadth (cubical epithelium). 

 Stratified epithelium consists of several layers of cells, or, to 



