THE SKIN AND SKELETON. 197 
epidermis. It is that part of the skin which is raised in a 
blister. It is thickest where there is most pressure or hard 
usage: on the back of the Camel it attains unusual thick- 
ness. The lower portion of the epidermis (called rete mu- 
Fie. 94.—Section of Skin, from Horse’s Nostril: E, epidermis; D, derma; 1, horny 
layer of epidermis; 2, rete mucosum; 8, papillary layer of derma; 4, excretory 
duct of a sudoriparous, or sweat, gland; 5, glomerule, or convoluted tube of the 
same; 6, hair follicle; 7, sebaceous gland; 8, internal sheath of the hair follicle ; 
9, bulb of the hair; 10, mass of adipose tissue. 
cosum) is comparatively soft, and consists of nucleated 
cells containing pigment-granules, on which the color of 
the animal depends. ‘Toward the surface, the cells be- 
come flattened, and finally, on the outside, are changed to 
horny scales (Fig. 2, ¢). 
These scales, in the higher animals, are constantly wear- 
ing off in the form of scurf, and as constantly being re- 
uewed from below. In Lizards and Serpents, the old epi- 
dermis is cast entire, being stripped off from the head to 
the tail; in the Toad, it comes off in two pieces; in the 
Frog, in shreds; in Fishes and some Mollusks, in the form 
of slime. However modified the epidermis, or whatever 
its appendages, the like process of removal goes on. Mam- 
mals shed their hair; Birds, their feathers; and Crabs, 
