EPITHELIUM. 



varies much, in different parts, in thickness. Its outer portion con- 

 stitutes the cuticle, and its inner part the rete mucosum, or stratum 

 Malpighii, of anatomists. 



The outer layers of cells in case of the mucous membrane also 

 become flattened into coherent scales (the epithelial plates). These 

 may be detached in flakes or sheets from the oesophagus, and are 

 often so by disease, from the tongue. (Fig. 147.) The epithelium 



Fig. 147. 



Epithelial plates of oral cavity, a. Large, b. Middle-sized, c. Same, with two nuclei. Magnified 

 350 diameters. (KoUiker.) 



upon this organ is sometimes even $ of an inch thick. Still, it is 

 very endosmotic, various fluids penetrating it from without, and 

 the blood-plasma also exuding through it from the vessels which 

 underlie it. (Kb'lliJcer.) 



In the lower animals we find various modifications of this epithe- 

 lium as in the sheaths of the beaks of birds and of Chelonian 

 reptiles; in the scales of fishes; in the jaws of certain invertebrate 

 animals ; in whalebone so-called, tortoise-shell, and the teeth of some 

 fishes; in the spines and plates of the tongues of many animals, 

 and the spines of the oesophagus of the Chelonia ; in the teeth-like 

 appendages of the stomachs of some of the mollusca, and the horny 

 plates of the gizzards of most birds, and of the cardiac half of the 

 stomach of the horse. 



II. CONOIDAL EPITHELIUM. 

 A. Simple Conoidal Epithelium. 



This variety of epithelium, consisting of a single layer of conoidal 

 cells, is represented by Figs. 148, 149, and 150. 



Distribution. It commences at the cardiac orifice of the stomach, 

 16 



