THE EPITHELIAL TISSUES 23 



being caudate, in the latter fusiform, or they may be greatly elongated so as to 

 become fibers. Cells with hair-like processes, or cilia, projecting from their 

 free surfaces, are a special variety. The cilia vary greatly in size, and may 

 even exceed in length the cell itself. Finally, cells may be branched or stellate 

 with long outstanding processes. 



From theirfunction cells are called secreting, protective, sensitive, contractile, 

 and the like. 



From their origin cells are called epiblastic and mesoblastic and hypoblastic, 

 ectodermic, mesodermic, and endodermic. 



Modes of Cell Connection. Cells are connected together to form tissues 

 in various ways. They are connected by means of a cementing intercellular 

 substance. This is probably always present as a transparent, colorless, viscid, 

 albuminous substance, even between the closely apposed cells of epithelium; 

 while in the case of cartilage it forms the main bulk of the tissue, and the cells 

 only appear as embedded in, not as cemented together by, the intercellular 

 substance. This intercellular substance may be either homogeneous or 

 fibrillated. In many cases, e.g., the cornea, it can be shown to contain a 

 number of irregular branched cavities, which communicate with each other, 

 and in which branched cells lie. Nutritive fluids can find their way through 

 these branching spaces into the very remotest parts of a non-vascular tissue. 

 The basement membrane, membrana propria must be mentioned as a special 

 variety of intercellular substance which is found at the base of the epithelial 

 cells in most mucous membranes, and especially as an investing tunic of 

 gland follicles which determines their shape. 



Cells are connected by anastomoses of their processes. This is the usual 

 way in which stellate cells, e.g., of the cornea, are united. The individuality 

 of each cell is thus to a great extent lost by its connection with its neighbors 

 to form a reticulum. As an example of a network so produced we may cite 

 the anastomosing cells of the reticular tissue of lymphatic glands. 



The intercellular substance sometimes forms so great a part of the tissue 

 as to overshadow the cells proper. Examples of this type of structure are 

 found in the matrix of cartilage, the fibers of connective tissue, bone, etc. 



Decay and Death of Cells. There are two chief ways in which the 

 comparatively brief existence of cells is brought to an end, i.e., by mechanical 

 abrasion and by chemical transformation. 



The various epithelia furnish abundant examples of mechanical abrasion. 

 As it approaches the free surface, the epidermal cell becomes more and more 

 flattened and scaly in form and more horny in consistency, till at length it is 

 simply rubbed off. Hence we find free epithelial cells in the mucus of the 

 mouth, in the intestine, and in the genito-urinary tract, as well as on the sur- 

 face of the outer skin. 



In the case of chemical transformation the cell contents undergo a 

 degeneration which, though it may sometimes be pathological, is very often 



