CHAPTER VI. THE TISSUES. 



lig. 8. CILIATED COLUMNAR 

 EPITHELIUM, 



Section perpendicular to free surface. 



1. Classification of Tissues. The tissues of the rabbit's 

 body can be arranged in four groups (1) Epithelial ; (2) 

 Connective ; (3) Muscular ; and (4) Nervous. Of these, 

 the last two are mainly 

 concerned in doing work, and 

 in them, therefore, katabolism 

 is most active ; the cells of 

 the second are mainly con- 

 cerned in forming cell-pro- 

 ducts, especially plastic pro- 

 ducts ; while the epithelial 

 cells are most concerned with secretion, though they may 

 do mechanical work (ciliated epithelium), or be purely 

 passive (endothelium). 



2. Epithelial Tissues consist entirely or almost entirely 

 of cells, a small amount of ectoplastic material cementing 

 the cells firmly together. Epithelial tissues do not usually 

 attain any great thickness, but form extensive thin sheets, 

 often consisting of one layer of cells only, and abutting on 

 some space. Thus the epidermis covering the surface of the 



body, the lining 

 of the alimentary 

 canal, the lining of 

 the body cavity, 

 of the blood-vessels, 

 of all ducts and 

 tubes all these are 

 epithelia. 



According to the form of the constituent cells we 

 may roughly classify epithelia into squamous and columnar. 

 In the former the cells are flattened, their flat surfaces 

 being parallel to the surface of the epithelium (fig. 9) ; 

 in the latter the cells are lengthened in a direction 



47 



->> / ^ -^^ . B 



Fig. 9. SQUAMOUS EPITHELIUM. 



A, Ordinary Overlapping ; B, Pavement. Sections 

 perpendicular to free surface. Diagrammatic. 



