EXCRETION 
459 
excretion. In order to understand just how wastes are 
removed by the skin, it is necessary to study its parts. The 
outer layer of the skin is called the epidermis (Figure 409) 
and is chiefly composed of dead cells. These outer cells are 
constantly breaking off, a process which is most apparent 
in the case of sunburn. Whatever pigment or coloring 
matter there is in the skin is located in the inner cells of the 
epidermis. The amount and kinds of pigment determine 
whether a person is of light or dark complexion, white, black, 
or yellow. These inner 
cells are constantly grow¬ 
ing new cells to replace 
the cells which scale off. 
The nails and the hair 
arise in the outer layer of 
the skin. Other structures 
which arise in the same 
way are the scales of fishes 
and snakes, the hoofs and 
horns of cattle, and the 
feathers of birds. 
The inner layer of the 
skin is the dermis; it con¬ 
tains blood vessels, nerves, 
connective tissue, the sweat glands, and sense organs of 
touch, pain, heat, and cold. It is estimated that there are 
over two million sweat glands in the skin of man. These 
are the excretory organs of the skin and their work is to 
eliminate waste substances from the blood and to keep 
the body temperature normal (98.4° F.) by regulating the 
amount of perspiration excreted. The amount of perspira¬ 
tion is influenced both by the temperature of the body and 
the air. The evaporation of perspiration keeps the body 
at the normal temperature. 
The skin is attached to the body by a loose layer of con- 
Figure 409 . — Diagram of Skin. 
