32 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



Functions of Epithelium. According to function, 

 epithelial cells may be classified as: i, protective, e.g., in 

 the skin, mouth, blood vessels, etc. ; 2, protective and motive, 

 ciliated epithelium; 3, secreting, glandular epithelium; 4, 

 germinal, as epithelium of testicle producing spermatozoa; 

 5, absorbing and secreting, e.g., epithelium of intestine; 6, 

 sensory, e.g., olfactory cells, organ of Corti. 



Epithelium forms a continuous smooth investment 

 over the whole body, being thickened into a hard, horny 

 tissue at the points most exposed to pressure, and develop- 

 ing various appendages, such as hairs and nails. Epithe- 

 lium lines also the sensorial surfaces of the eye, ear, nose, 

 and mouth, and thus serves as the medium through which 

 all impressions from the external world touch, smell, 

 taste, sight, hearing reach the delicate nerve endings, 

 whence they are conveyed to the brain. 



The ciliated epithelium which lines the air passages 

 serves to promote currents of the air in the bronchial tubes 

 and to propel fluids and minute particles of solid matter 

 out of the body. In the case of the Fallopian tube, the 

 cilia assist the progress of the ovum toward the cavity of 

 FIG. 36. Cili- the uterus. 

 Intestine of a The epithelium of the various glands, and of the whole 



Mollusk. (En- intestinal tract, has the power of secretion, i.e., of produc- 

 gelmann.) . , . -11 < , i T 



ing certain materials by processes of metabolism m its 



protoplasm. 



Epithelium is likewise concerned in the processes of transudation, 

 diffusion, and absorption. 



II. THE CONNECTIVE TISSUES. 



This group of tissues forms the skeleton with its various connections 

 bones, cartilages, and ligaments and also affords a supporting framework 

 and investment to the various organs composed of nervous, muscular, and 

 glandular tissue. Its chief function is the mechanical one of support, and 

 for this purpose it is so intimately interwoven with nearly all the textures of 

 the body that if all other tissues could be removed, and the connective tissues 

 left, we should have a wonderfully exact model of almost every organ and 

 tissue in the body. 



General Structure of Connective Tissue. The connective tissue is 

 made up of two chief elements, namely, cells and intercellular or formed 

 substance. 



Cells. The cells are usually of an oval shape, often with branched 

 processes, which are united to form a network. They are most readily 



