42 



GENERAL ANATOMY. 



the main purpose served by the epithelium (here called the epidermis) is that of 

 protection. As the surface is worn away by the agency of friction or change of 

 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 ensure a 

 perfectly smooth surface. 



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

 divide these several epithelial linings into two classes : into (1) epithelial tissue 

 proper, consisting 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 squamous cells, joined edge to edge in such a man- 

 ner 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 lym- 

 phatics ; on the surface of the brain and spinal cord, and in the anterior chamber 

 of the eye. And, though the separation must be an artificial one, since every 

 gradation of transition between the two classes may be observed, it would seem 

 advisable for the purpose of description to employ it. 



1. True epithelial tissue consists of one or more layers of cells, united together 



FIG. 7. Epithelial cells from the oral cavity of man. Magnified 350 times, a. Large, b. Middle-sized, 

 c. The same with two nuclei. 



by an interstitial cement-substance, supported on a basement-membrane, and is 

 naturally grouped into two classes, according as there is a single layer of cells (simple 

 epithelium) or more than one (stratified epithelium). The various kinds of 

 epithelium, whether arranged in a single layer or in more than one layer, are 

 usually spoken of as squamous or pavement, columnar, spheroidal or glandular, 

 ' and ciliated. 



The pavement epithelium (Fig. 7) is composed of flat nucleated scales of vari- 

 ous 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 flat- 

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

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

 work, which gives to the cell the appearance of granulation. This kind of epi- 

 thelium is found on the surface of the skin (epidermis) and on mucous surfaces 

 which are subjected to friction. The nails, the hairs, and (in animals) the horns 

 are a variety of this kind of epithelium. 



A variety of squamous epithelium which is found in the deeper layers of 



