THE EPITHELIUM. 



1083 



temperature new cells are supplied, and thus the surface of the true skin and the 

 vessels and nerves which it contains are defended from damage. In the gastro- 

 intestinal mucous membrane and in the glands the epithelial cells appear to be 

 the principal agents in separating the secretion from the blood or from the aliment- 

 ary fluids. In other situations (as the nose, fauces, and respiratory passages) the 

 chief office of the epithelial cells appears to be to maintain an equable tempera- 

 ture by the moisture with which they keep the surface always slightly lubricated. 

 In the serous cavities they also keep the opposed layers moist, and thus facilitate 

 their movements on each other. Finally, in all internal parts they insure a 

 perfectly smooth surface. 



Of late years there has been a tendency on the part of many histologists to 

 divide these several epithelial structures into two classes : (1) epithelium, consist- 

 ing of nucleated protoplasmic cells, which 

 form continuous masses on the skin and 

 mucous surfaces and the linings of the ducts 

 and alveoli of secreting and excreting glands ; 

 and (2) endothelium, which is composed of a 

 single layer of flattened transparent squam- 

 ous cells, joined edge to edge in such a 

 manner as to form a membrane of cells. This 

 is found on the free surfaces of the serous 

 membranes, as the lining membrane of the 

 heart, blood-vessels, and lymphatics ; on the 

 surface of the brain and spinal cord, and in 

 the anterior chamber of the eye. Endothelium originates from the embryonic 

 mesoblast, while epithelium arises, as a rule, from the epiblast or hypoblast. 



Epithelium consists of one or more layers of cells united together by an inter- 

 stitial cement-substance, supported on a basement-membrane, and is naturally 

 grouped into two classes, according as to whether there is a single layer of cells 

 (simple epithelium) or more than one (stratified epithelium}. A third variety 

 (transitional epithelium] is that in which cells in three or four layers are so fitted 

 together that the appearance is not one of distinct stratification. The different 

 varieties of simple epithelium are usually spoken of as squamous or pavement, 

 columnar, glandular or spheroidal, and ciliated. 



The pavement epithelium (Fig. 602) is composed of flat nucleated scales of 

 various shapes, usually polygonal, and varying in size. These cells fit together 

 by their edges, like the tiles of a mosaic pavement. The nucleus is generally 

 flattened, but may be spheroidal. The flattening depends upon the thinness of 

 the cell. The protoplasm of the cell presents a fine reticulum or honeycombed 



Striated free borders 

 of cells. 



FIG. 602. Simple pavement epithelium. 



FIG. 603. Columnar epithelium from an intestinal villus. 



FIG. 604. Goblet cells. (From 

 Kirk's Physiology.) 



network, which gives to the cell the appearance of granulation. This kind of 

 epithelium forms the lining of the air-cells of the lungs. The endothelium, 

 which covers the serous membranes, and which lines the heart, bloodvessels, 

 lymphatics, and the anterior chamber of the eye, is also of the pavement type. 

 The columnar or cylindrical epithelium (Fig. 603) is formed of cylindrical or 

 rod-shaped cells set together so as to form a complete layer, resembling, when 

 viewed in profile, a palisade. The cells have a prismatic figure, more or less 



