24 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



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). 



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



(1) Squamous or pavement. Cells flattened. 



(a) Non-ciliated. Alveoli of lungs, also includes endothelium, 

 lining the blood vessels, and mesothelium, lining the large 

 serous spaces. 



(b) Ciliated. The peritoneum of some forms at breeding season. 



(2) Cubical epithelia. Cells with the three dimensions approxi- 



mately equal, mainly glandular. 



(a) Non-ciliated. The usual type. It is found lining both 

 ducts and secretory portions of most glands, the pigmented 

 layer of the retina, etc. 



(b) Ciliated. Not common. Lining of some of the smaller 

 bronchial tubes. 



(3) Columnar. Cells may be cylindrical, conical, or goblet-shaped. 



(a) Non-ciliated. Intestinal. 



(b) Ciliated. Fallopian tube and uterus. 



(c) Pseudo-stratified. Smaller bronchi, nasal duct, etc. 



2. Stratified epithelia. Cells more than one layer in thickness, 

 (i) Squamous. Surface cells flattened. 



(a) Non-ciliated. Skin, mouth, vagina, etc. 



(b) Ciliated. Pharynx of embryo. 



