3 2 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



According to some observers, these processes are connected with longitudinal 

 fibers which pass to the other end of the cell, but which are not connected 

 with the nucleus. 



FIG. 34. FIG. 35. 



FIG. 34. Spheroidal Ciliated Cells from the Mouth of the Frog. X 300 diameters. 

 (Sharpey.) 



FIG. 35. Ciliated Epithelium from the Human Trachea, a, Large, fully formed cell; 

 ft, shorter cell; c, developing cells with more than one nucleus. (Cadiat.) 



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 

 the uterus. 



The epithelium of the various glands, and of the 

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



FIG. 36. Cili- 

 ated Cell of the 

 Intestine of a 

 Mollusk. 



gelmann.) 



producing certain 



ism in its protoplasm. 



materials by processes of metabol- 



