Epithelial Tissues. 



21 



Epithelial Tissues. 



Epithelial tissues are distinguished chiefly as surface invest- 

 ments, such as those of the exterior of the body, the interior of the 

 alimentary canal, the lungs, the respiratory and accessory respira- 

 tory tracts, and the ducts of the urinogenital organs. In all 

 epithelia the cellular feature is a prominent one, and it is largely 

 for this reason that as lining membranes they are not conspicuous 

 in gross structure. They are noteworthy, however, for their 

 products, the hairs and the various 

 kinds of secreting organs or glands. 



The epithelium of the skin (Fig. 2) 

 is known as the epidermis or scarf-skin. 



It is stratified, i.e., composed of 

 several layers deep of cells, of which the 

 deeper are formative, growing cells, 

 while those at the surface are flattened 

 squamous cells, and are successively 

 discarded. The several layers combined 

 produce but a thin membrane. It 

 extends over the entire surface of the 

 body and connects at certain points 

 with the epithelia of the internal sur- 

 faces. It is supported by a thick 

 resistant layer of connective tissue which 

 forms the true skin or corium. 



The epithelium of the internal sur- 

 faces forms the chief portion of the 

 mucous membranes. In the greater 

 portion of the alimentary tract the 

 epithelial layer is simple or one layered and is associated with a thin 

 layer of smooth muscle to form a mucous tunic (Fig. 16, t.ms.). 



The coating of hairs on the surface of the body, the presence 

 of which is a notable mammalian feature, is a protective invest- 

 ment arising from the epidermis. A hair is produced by the 

 modification of the central portion of an ingrowth of the epidermis, 

 termed the hair follicle (Fig. 2, f). The latter contains at its 

 base a small elevation of the underlying vascular connective tissue, 

 the hair papilla, through which the structure is nourished. On 



Fig. 2. From a section of the 

 upper lip of a four-day-old rabbit, 

 a, a', erector muscles of the hair; 

 c, connective tissue of corium; e, 

 epidermis; f, hair follicle: g, seba- 

 ceous gland; p, papilla; s, hair shaft. 



