24 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



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. 



As the cells approach decay and death their normal physiological processes 

 diminish in intensity and finally cease. This occurs early in the transforma- 

 tion and function is lost before the cell form is destroyed beyond recognition. 



THE STRUCTURE OF THE ELEMENTARY TISSUES. 



In the differentiation of the protoplasm of the body, great masses of 

 cells are formed of the same elemental structure and typical functional prop- 

 erties. These are the elementary tissues. The tissues alone or in combina- 

 tion in varying proportions constitute the organs of the body. These ele- 

 mentary 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 men- 

 struum, 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 an internal free surface, together with the cellular portions of the 

 glands which are connected with, or developed from, these free surfaces. 



Epithelium clothes (i) the whole exterior surface of the body, forming 

 the epidermis with its appendages; becoming continuous at the chief orifices 

 of the body nose, mouth, anus, and urethra with (2) the epithelium which 

 lines the whole length of the respiratory, alimentary, and genito-urinary 

 tracts, together with the ducts and secretory cells of their various glands. 

 Epithelium also lines the cavities of (3) the brain and the central canal of the 

 spinal cord, (4) the serous and synovial membranes, and (5) the interior of 

 blood vessels and lymphatics. 



Epithelial cells vary in size and shape, pressure being the main factor in 

 this variation. The protoplasm may be granular, reticular, or fibrillar in 

 appearance. The nucleus is spherical or oval, usually there is only one, but 

 there may be two or more present. 



Epithelial tissues are non-vascular, that is to say, do not contain blood 

 vessels, but in some varieties minute channels exist between the cells of 

 certain layers. Nerve fibers are supplied to the cells of many epithelia. 



CLASSIFICATION OF EPITHELIA. 

 As to form and arrangement of cells. 



I. Epithelia in the form of membranes (covering surfaces), 

 i. Simple epithelium. Cells only one layer in thickness. 



