GENERAL ANATOMY OF TEE EPITHELIUMS. 185 



epithelium, particularly that of the digestive organs, is thus 

 shown by its early formation ; its dimensions, in the embryo, 

 are immense. We find that, by the thickness of its layers, 

 it blocks up the opening of the small intestine in the fcetus, 

 and even in the adult it is sometimes 4 or 5 times thicker 

 than the membrane which supports it. 



I. GENERAL ANATOMY OF THE EPITHELIUMS. 



ANATOMISTS recognize two distinct forms of epithelium, 

 pavement and columnar epithelium; it is only in their 

 extremes, however, that they differ so much, there being inter- 

 mediate forms between them. The principal epithelium, for 

 instance, that which forms the essential parenchyma of the 

 glands, is neither the pavement nor the columnar epithelium; 

 it is a kind of spherical globule. 



The membranes, whose free surface is coated with epithe- 

 lium, belong to two categories : 1, serous membrane, generally 

 forming closed cavities ; 2, integumentary membrane (either 

 internal or external). The distinguishing characteristics ob- 

 served in these membranes are dependent on the nature of 

 their epithelium. 



A. Serous Membrane. 



The class of epithelium spread on the surface of the 

 serous membranes, is the pave- 

 ment form (Fig. 56, A). It is A 

 generally a single layer of cells c^ir^~> ^~r > a g -^rsz 

 which, in consequence of recip- 

 rocal deformation (being crowd- 

 ed together), have flattened into 

 angular, polygonal disks : such 

 is the epithelium of the abdom- 

 inal serum ; the case is the same 

 with that of the pericardium, of 

 the arachnoid membranes, and of 

 all the serous membranes called 

 visceral. The epithelium which 

 lines the inner surface of the 

 blood-vessels, and the cavities of the heart (endocardium) is 

 also of this kind. The epithelium covering the articulating 

 cavities is also pavement, but composed of several layers; 



* A, Pavement epithelium. B, Columnar epithelium. C. Stratified epithe- 

 lium. 





Fig. 56. 

 Various forms of epitheliums.* 



