22 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



cells lie. Nutritive fluids can find their way through these branching spaces 

 into the very remotest parts of a non-vascular tissue. The basement mem- 

 brane (membrana propria) must be mentioned as a special variety of intercellu- 

 lar 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 deter- 

 mines their shape. 



Cells are connected by anastomosis 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. 



Derived Elements. Besides the cell, which may be termed the 

 primary tissue element, there are materials which may be termed secondary 

 or derived elements or formed materials. 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, by mechanical abra- 

 sion and by chemical transformation. 



The various epithelia furnish abundant examples of mechanical abrasion. 

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

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

 off as in the epidermis. Hence we find free epithelial cells in the mucus of 

 the mouth, intestine, and in the genito-urinary tract. 



In the case of chemical transformation the cell-contents undergo a degener- 

 ation which, though it may sometimes be pathological, is very often a normal 

 process. Thus we have cells by fatty metamorphosis producing oil-globules 

 in the secretion of milk, fatty degeneration of the muscular fibers of the uterus 

 after the birth of the fetus. Calcareous degeneration is common in the cells of 

 many cartilages. 



THE STRUCTURE OF THE ELEMENTARY TISSUES. 



There are certain elementary structures formed in the process of differentia- 

 tion which alone or when combined in varying proportions form the whole 

 of the organs and tissues of the body. These elementary tissues are: The 

 Epithelial, The Connective, The Muscular, and The Nervous Tissues. To 

 these four some would add a fifth, looking upon the Blood and Lymph, con- 

 taining, as they do, formed elements in a fluid menstruum, as a distinct tissue. 



I. THE EPITHELIAL TISSUES. 



Epithelium is a tissue composed almost wholly of cells, with a very small 

 amount of intercellular substance which glues the cells together. In general 

 it includes all those cellular membranes which cover either an external or 



