ast three kinds, as follows:

1. _From mechanical injuries_ such as might result from contact with hard,
rough, or sharp objects. The main quality needed for resisting mechanical
injuries is _toughness_, and this is supplied both by the epidermis and by
the connective tissue of the dermis.

2. _From chemical injuries_ caused by contact with various chemical
agents, as acids, alkalies, and the oxygen of the air. The epidermis,
being of such a nature as to resist to a considerable extent the action of
chemical agents, affords protection from these substances. (89)

3. _From disease germs_ which are everywhere present. The epidermis is the
main protective agent against attacks of germs, but should the epidermis
be broken, they meet with further resistance from the fluids of the dermis
and the white corpuscles of the blood.

4. _From an excessive evaporation of liquid from the surface of the body_.
In the performance of this function, the skin is an important means of
keeping the tissues soft and the blood and lymph from becoming too
concentrated.

*Other Functions of the Skin.*--Through the perspiratory glands the skin is
an _organ of excretion_. While the secretion from a single gland is small,
the waste that leaves the body through all of the perspiratory glands is
considerable (90) (page 206). By means of the nerves terminating in the
touch corpuscles, the skin serves as the _organ of touch_, or feeling
(Chapter XX). To a slight extent also the skin may absorb liquid
substances, these being taken up by the blood and lymph vessels, and
perform a respiratory function, throwing off carbon dioxide. But the most
important function of the skin, in addition to protection, is that of
serving as

*An Organ of Adaptation.*--Forming, as it does, the boundary between the
body and its physical environment, the skin is perhaps the most important
agent through which the body is adapted to its immediate surroundings.
Evidence of this is found in the great variety of influences which are
able to affect the body through their action upon the nerves in the skin,
and in the changes which the epidermis undergoes on exposure. The latter
function is especially marked in the lower animals, the coverings of
epidermal tissue (hair, scales, feathers, etc.) adapting each species to
the physical conditions under which it lives. In man the most striking
example of adaptation through the skin is seen in the variations in the
quantity of blood c