STRUCTURAL AFFINITIES. 209 



suits, rather by modifying parts already formed than 

 by creating anew. 



The mucous membranes may be considered as in- 

 ternal prolongations of the skin. The cells of the 

 cuticle of the skin are colorless and flattened, often 

 wrinkled and folded, and correspond to the pave- 

 ment or tessellated epithelium of the mucous mem- 

 brane. Subjacent to the epithelium or epidermis, 

 there occurs a structureless basement membrane, 

 which can rarely be demonstrated on account of its 

 extreme tenuity. The third layer of the mucous 

 membrane, corresponding to the cutis vera of the 

 skin, is also composed of areolar and elastic tissues, 

 and in both is highly vascular, and furnished with 

 papillce or villi. These three structures in both are 

 supported by a layer of lax tissue, in which the 

 areoloe frequently contain fat. 



Glands themselves are of an epithelial nature, and 

 are but adjuncts of the skin. As Virchow explains 

 it, an epithelial cell begins to divide, and goes on 

 dividing again and again, until by degrees a little 

 process composed of cells grows inward, and spread- 

 ing out laterally gives rise to the development of 

 a gland, which thus straightway constitutes a body 

 continuous with layers of cells originally external. 

 Thus arise the glands of the surface of the body* the 

 sudoriferous and sebaceous glands of the skin, and 

 the mammary gland. 



If, then, we could unravel the milk glands, so as to 

 present the interior surface flat, but little change ex- 

 cept th;it of adaptation would be required to identify 

 10* 



