62 NOKMAL HISTOLOGY. 



ering the skin, mouth, tongue, lower half of pharynx, oesophagus, 

 epiglottis, upper part of larynx, pelvis of kidney, ureter, bladder, 

 beginning and end of male urethra, and the whole female urethra. 



The deeper layers of this tissue have cells more nearly spherical, 

 which are often connected with each other by slender processes. 

 These procesess give rise to the so-called prickle cells of the deeper 

 layers of the epidermis. The cells of the outer layer become much 

 flattened., lose their protoplasm, and by constant friction are worn 

 away and cast off. This process is called desquamation. 



Columnar epithelium. This form of epithelial tissue is consti- 

 tuted of cells which are colummar in shape as seen from the side, 

 but from above they appear hexagonal. The first layer, resting 

 upon the membrana propria, consists of spherical cells, but those 

 of the next layer are oval. Simple colummar epithelium is found 

 lining the mucous membrane of the alimentary tract from the car- 

 diac orifice downward. The stratified variety is found in the ex- 

 cretory ducts of glands leading into the alimentary tract and in a 

 portion of the male urethra. This tissue is of hypodermic origin. 



Ciliated epithelium, This resembles columnar epithelium, but 

 the outermost layer of cells is provided with cilia. Cilia are delicate 

 protoplasmic projections which by their motion produce outward 

 currents of mucus and other products. The cells are nucleated and 

 rest upon a basement membrane. Ciliated epithelium occurs in the 

 nasal cavities, the Eustachian tubes, larynx, trachea, bronchi, a por- 

 tion of the uterus, Fallopian tubes, vasa efferentia (partly), the ven- 

 tricles of the brain, and the central canal of the spinal cord. 



Modified epithelium. This is represented by modifications of 

 the types given above. The important varieties are goblet cells, 

 pigmented epithelium, and transitional epithelium. 



Goblet cells are modifications of columnar or ciliated cells. Each 

 cell is generally isolated from others of like character and is formed 

 by the elaboration of mucin from the protoplasm, which so fills up 

 the cell as to cause it to become swollen and elliptical in. shape. 

 Eventually the cell bursts, discharging its contents upon the surface 

 of the membrane. This is one source of mucus, and hence the term 

 mucous membrane. 



Pigmented epithelium. This is represented by cells of the squa- 



