PROTECTIVE AND EXCRETORY FUNCTIONS OF THE SKIN. 413 
standing of this relation even to details makes the practice of 
medicine more scientific and practically effective, and gives 
physiology greater breadth. 
The skin has a triple function: it is protective, excretory, 
sensory, and, we may add, nutritive (absorptive) and respira- 
tory, especially in some groups of animals. 
As a sensory organ, the skin will receive attention later. 
Protective Function of the Skin.—Comparative.—Among many 
groups of invertebrates the principal use of the exterior cover- 
ing of the body is manifestly protection. Among these forms, 
an internal skeleton being absent, the exo-skeleton is developed 
externally, and serves not only for protection, but for the at- 
tachment of muscles, as seen in crustaceans and insects. But 
this part of the subject is too large for detailed treatment in 
such a work as this. Turning to 
the vertebrates, we see scales, 
bony plates, feathers, spines, hair, 
etc., most of them to be regarded 
as modifications of the epidermis, 
always useful, and frequently also 
ornamental. 
Primitive man was probably 
much more hirsute than his mod- 
O RO ere 
Fig. 321. 
Fig. 321.—Sudoriparous glands. 1 x 20(After Sappey.) 1,1, epidermis ; 2,2, mucous layer ; 
8, 3, papille; 4, 4, derma; 5, 5, subcutaneous areolar tissue ; 6, 6, 6, 6, sudoriparous 
glands ; 7,7, adipose vesicles; 8.8, excretory ducts in derma; 9, 9, excretory ducts divided. 
Fia. 323.—Portion of skin of palm of hand about one-half an inch (127 mm.) square. 1 x 4, 
(After Sappey.) 1, 1, 1, 1, openings of sudoriferous ducts; 2, 2, 2, 2, grooves between 
papillae of skin. 
ern representative; and, though the human subject is at pres- 
ent provided with a skin in which protective functions are at 
their lowest, still the epidermis does serve such a purpose, as 
