CONNECTIVE TISSUES 



31 



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 mo- 

 tive, ciliated epithelium; 3, secreting, glandular epithelium ; 

 4, germinal, as epithelium of testicle producing sperma- 

 tozoa; 5, absorbing and secreting, e.g., epithelium of intes- 

 tine; 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. Epi- 

 thelium lines also the sensorial surfaces cf 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 end- 

 ings, 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 the 



UterUS. FIG. 36. Ciliated 



Cell of the Intestine 



The epithelium of the various glands, and of the of a Moiiusk. (En- 



gelmann.) 



whole intestinal tract, has the power of secretion, i.e., of 

 producing certain materials by processes of metabolism in 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 glan- 

 dular 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, an4 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 sub- 

 stance. 



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

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



