414 ANIMAL PHYSIOLOGY. 
all have some time realized when it has been aeetny re- 
moyed by blistering, etc. 
Taking the structure of the skin of man as representing that 
of mammals generally, certain points claim attention from the 
Fic. 323.—Hair and _ hair-follicle (after Sappey). 1, 
root of hair ; 2, bulb of hair ; 3, internal root- 
sheath ; 5, membrane of hair- follicle ; 6, external 
membrane of follicle; 7, 7, muscular bands at- 
tached to follicle ; 8, 8. extremities of bands pass- 
ing to skin; 9, compound sebaceous gland, with 
duct (10) opening into upper third of follicle: 11, 
ae sebaceous gland; 12, opening of hair- fol: 
cle 
physiologist. Its elastic- 
ity, the failure of which 
in old age accounts for 
wrinkles ; its epidermal 
covering, made up of 
numerous layers of cells; 
its coiled and_ spiral- 
ly twisted sudoriferous 
glands, permitting of 
movements of the skin 
without harm to these 
structures; its hair-folli- 
cles and associated seba- 
ceous glands, the fatty 
secretion of which keeps 
the hair and the skin gen- 
erally soft and pliable. 
The muscles of the 
skin, which either move 
it as a whole or erect in- 
dividual hairs, play an 
important part in modi- 
fying expression, well 
seen in the whole canine 
tribe and many others. 
There are several mod- 
ifications of the sebaceous 
glands that furnish high- 
ly odoriferous secretions, 
as-in the civet cat, the 
skunk, the musk - deer, 
and many lower verte- 
brates. In some, these 
are protective (skunk) ; 
in others, though they 
may not be agreeable to 
the senses of man, they are doubtless attractive to the females 
of the same tribe, and are to be regarded as important in 
“ sexual selection.” being often confined to the males alone. 
