THE EPITHELIAL TISSUES 23 



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. 



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 

 abrasion 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 

 degeneration w r hich, 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 dif- 

 ferentiation 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, 

 containing, 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 



