EPITHELIAL TISSUES 13 



the fertilized egg, in development toward the adult condition, gives 

 rise by division of labour to a great variety of cells, each kind of 

 which may be regarded as representing a minor aspect of some 

 major function. 



The Tissues 



The primary tissues of the body are of four kinds— epithelial, 

 connective, muscular, and nervous. To these — the fixed tissues — 

 are to be added the fluid substances, blood and lymph, in which 

 the cell elements, the red and white corpuscles, or in the latter 

 case the white elements alone, are suspended in a fluid medium. 

 The differences between these depend partly upon the characters 

 of the cells composing them and partly upon the nature and 

 quantity of the material between the latter, the intercellular matrix. 



The following survey of the principal features of the tissues will 

 serve to make clear the extent to which the gross appearance of 

 organs depends upon tissue composition, though the account is in 

 no way intended as a guide to the microscopic structure of the rabbit 

 which is more properly part of the subject-matter of histology. 



1. Epithelial Tissues 



Epithelial tissues are distinguished chiefly as surface invest- 

 ments, such as those of the exterior of the body, and those of the 

 interior of the alimentary canal, the lungs, the respiratory and 

 accessory respiratory tracts, and the ducts of the urinogenital 

 organs. In all epithelia the cellular feature is a prominent one, 

 the amount of intercellular material being relatively small. With 

 a few exceptions, they are not penetrated by blood-vessels. As 

 constituents of lining membranes, they are not conspicuous in 

 gross structure but they give rise to important derivatives, such as 

 the hairs and the various kinds of secreting organs or glands. 



Epithelium may be simple, i.e. one-layered, or it may be strati- 

 fied, i.e. composed of several layers of cells. The cells composing 

 it may be flattened or squamous, cuboidal or isoprismatic, or col- 

 umnar and are packed together so closely that in free surface view 

 each is seen to be pressed into a more or less hexagonal form. (See, 

 for example, the epithelium lining the collecting tubule of the kid- 

 ney in Fig. 73, p. 126.) The epithelium of the skin (Fig. 2), 



