CHAPTEE VIII. 



EPITHELIAL TISSUES OR EPITHELIUM. KERATIN. 

 CHITIN. PIGMENTS DEPOSITED IN THE EPI- 

 THELIAL STRUCTURES. CERTAIN OTHER ANIMAL 

 PIGMENTS. 



BY the term epithelium is designated a tissue, composed entirely 

 of cells, which covers the whole external surface of the body, and lines 

 the cavities which open externally. The term is generally held to 

 include also the tissue composed of a single layer of tesselated or 

 tile-like cells which lines the arteries, capillaries and veins, the serous 

 sacs and the lymphatics. This tissue to which the terms endothelium 

 or epithelioid tissue are more properly applied, as indicating that it 

 differs from epithelium in its development, in its characters, and in 

 the uses which it subserves, will be considered in this work in discussing 

 the chemistry of the so-called ductless glands and the lymphatics. 



Confining our attention to epithelium proper we might classify 

 it in various ways : firstly, according to the form and arrangement of 

 the cells of which it is composed : secondly, according to the regions 

 in which it occurs : thirdly, according to the mode in which it is 

 developed : fourthly, according to the chemical characters which it 

 possesses ; we shall not, however, strictly follow any of these modes 

 of classification. There is no other tissue of which the individual 

 anatomical elements exhibit such marked differences in the chemical 

 operations of which they are the seat. 



Speaking broadly we may, however, say that the epithelium 

 covering the external surface of the body is composed of cells which 

 are, even in their most active stages, the seat of but slow and 

 unimportant chemical changes, whilst a large number of them cease 

 to be the seat of any material exchanges whatever, or to manifest 

 any phenomena which characterize them as living, long before they 

 cease to form part of the Jiving body. 



The function of such epithelium and we are referring to that 

 which composes the cuticle and its appendages is in the strictest 

 sense tegumentary. This epithelium possesses two characters which 

 may be taken together. Firstly, it is entirely derived from the 

 external layer of the blastoderm or epiblast. Secondly, however 



