34 COMPARATIVE ZOOLOGY. 
flat or cylindrical cells, and in some parts (as in the wind- 
pipe of air-breathing animals, and along the gills of the 
Oyster) is covered 
with cilia, or minute 
hairs, about gz 55 of 
an inch long, which 
are incessantly movy- 
ing. Continuous 
with this inner lin- 
ing of the body (as 
seen on the lip), and 
Fic. 2.—Various kinds of Epithelium Cells: a, colum- covermg the outside, 
nar, from small intestine; 3, a single cell, showing j Ne fesexteraan! : 
nucleus; 0, ciliated, from one of the small air- os the epider MUS 1OA 
tubes; d, the same, from the windpipe, with single ey,¢7cle, It is the 
cell magnified about 200 times; c, squamous, from 
eyelid of a calf, showing changes of form, from the outer layer of the 
deep to superficial cells, 1 being the scurf. ° 5 
ory i 7 “skin,” which we 
can remove by a blister, and in Man varies in thickness 
from =}, of an inch on the cheek to +3, on the sole of the 
foot. It is constantly wearing off at the surface, and as 
constantly growing in the deeper portion; and in the proc- 
ess of growth and passage outward, the cells change from 
the spherical form to dead horny scales (seen in scurf and 
dandruff). In the lower layer of the cuticle we find the 
pigment cells, characteristic of colored races. Neither the 
epidermis nor the corresponding tissue within (epithelium) 
has any blood-vessels or nerves. The epithelial tissue, then, 
is simply a superficial covering, bloodless and insensible 
protecting the more delicate parts underneath. Hairs, 
horns, hoofs, nails, claws, corns, beaks, scales, tortoise- 
shell, the wings of Insects, etc., are modifications of the 
epidermis. 
(2) Connective Tissue—This is the most extensive tissue 
in animals, as it is the great connecting medium by which 
the different parts are held together. Could it be taken 
out entire, it would be a complete mold of all the organs. 
