^y() THE CELL AND MAMMALIAN HISTOLOGY 



CLASSIFICATION OF TISSUES 



Most tissues contain more than one type of cell, but almost 

 every tissue is made chiefly of one of the three main types that 

 we have described, and on this fact may be based a classification 

 of the tissues which is more rational than the traditional one. 

 Epithelium is tissue which is made predominantly of epithelio- 

 cytes ; it corresponds closely to epithelium defined in the old way 

 as tissue in which there is little ground substance, that is, material 

 formed outside the cells by the cells. Nervous tissue may be looked 

 on either as a subdivision or as a derivative of epithelium ; it 

 comes from epitheUum in development, but its characteristic cell, 

 the neurocyte, is different in appearance and properties from the 

 epitheliocyte, and will not divide when grown in tissue culture. 

 Mechanical tissue is based on the mechanocyte ; it includes con- 

 nective tissue, muscle, and the skeletal tissues, cartilage and 

 bone. Tissue based on amoebocytes has no obvious and simple 

 name, but may be called amceboid tissue if it is remembered that 

 it is not the tissue itself but its cellular constituents which are 

 amceboid. It corresponds roughly to what are also called the 

 fluids of the body. These three (or four) tissues and their 

 subdivisions will now be further described. 



EPITHELIA 



As has already been said, epitheUa are characterised by having 

 little ground substance, so that the cells are in close contact ; 

 microdissection has shown that they stick together by their 

 protoplasmic borders, and in some stained preparations inter- 

 cellular protoplasmic threads can be seen running from one cell 

 to the next. There is usually one, occasionally two, nuclei per 

 cell. EpitheUa generally form the surfaces of the body. They 

 are usually classified by the shape of the cells and the number of 

 layers, but although the names used must be known for descriptive 

 purposes this type of classification is not of fundamental import- 

 ance. The epitheUa of different parts of the body are speciaUsed 

 for different purposes, and experiments have shown that even 

 the epithelium of a given part of the external surface, such as 

 the flank, the sole of the foot, the cornea or a claw, retains its 

 specific appearance when grafted into another position. The best 

 classification of epitheUa wiU therefore be made by considering 



