EPITHELIAL TISSUES. Pall 
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 lin‘ng 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. 



2 
The epithelium of the internal sur- Fic. 2. From a section of the 
upper lip of a four-day-old rabbit. 
iicesmiomimlce tien chiel. portion. of ithe “2) “ercctor muscles of the han; 
c, connective tissue of corium; e, 
mucous membranes. In the greater epidermis; f, hair follicle; g, seba- 
ceous gland; p, papilla; s, hair shaft. 
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 
