The following are the chief duties of the skin :

1. To act as an organ of feeling (or touch).

2. To assist in removing carbonic acid and other noxious products from the body.

3. To give off perspiration, which is a fluid that consists of about ninety-nine parts of water and one part of various other substances. This secretion is called insensible perspiration.

When it cannot be seen on the skin by the unaided eyes; and sweat or sensible perspiration, when it is thus visible. The term, perspiration, is applied generally to both forms. The secretion of perspiration is regulated by the quantity of blood supplied to the skin, and also by direct stimulation of the nerves of the glands that secrete this fluid; both forms of control being exercised by the nervous system. Perspiration may be excited by muscular exercise, external heat, various poisons (including those of certain diseases), emotion, and other kinds of nervous disturbance, which may be indicative of bodily weakness or ill health. The drinking of water tends to promote perspiration by increasing the volume of the blood. On the other hand, as drinking water is generally much colder than the body, the ingestion of the former will as a rule lower the temperature of the latter. Hence, the warmer the water which is drunk, the more will the secretion of perspiration be excited; and vice versa. Speaking generally, we may say that the appearance of sweat is compatible with health only when it has been brought on by an increased supply of blood. When sweat breaks out independently of the circulation, the skin, as a rule, being free from an excess of blood and being cooled down by the process of evaporation that is in action on its surface, will be unusually cold; hence we may regard the existence of a "cold sweat" as a sign that the health of the animal is more or less upset. The hot and dry condition of the skin in certain stages of intermittent fever, for instance, is an example of the arrest of perspiration by a poison (toxin) produced by the presence of disease germs in the system.

We have seen (p. 12) that evaporation has a cooling effect on the surface from which it takes place; because a liquid to be converted into vapour, has to acquire a certain quantity of heat, which it naturally absorbs from its surroundings.

Evaporation is dependent on the capability of the air to absorb water. When air has absorbed all the moisture it can hold, it is said to be saturated, and then evaporation cannot go on. The degree of humidity in the atmosphere varies between the limits of absolute dryness and saturation point, neither of which are ever attained in nature. The degree of saturation varies according to the temperature; hence, the warmer the air, the more moisture it is capable of absorbing. Also, the drier the air, the more rapidly does evaporation take place, and consequently, the greater is its cooling effect. Dry atmospheric heat can therefore be far more easily borne than moist heat. The difference which the presence of moisture in the air makes on the rate of evaporation, is well exemplified by a comparison drawn between the condition of the skin of a man or horse in a hot moist climate like that of, say, Singapore, and in a hot dry one like that of the Arabian desert. In the former, the skin remains more or less wet from sweat which is unable to escape into the nearly saturated air; but in the latter, its usual condition is one of extreme dryness.

It is evident that the cleaner the skin and the shorter the coat, the more freely will evaporation take place from the surface of the body.

In addition to the evaporation of perspiration, there is an evaporation of moisture which passes through the skin mechanically, in the same way as it would do from a freshly killed dead body. This evaporation, which is far less than that from perspiration, has also of course a cooling influence on the system.

4. To secrete oil, which serves to lubricate the skin and hair. This oil is necessary to keep the skin in a pliable condition; for we find that when a large proportion of it has been removed, especially from those parts which are subjected to a good deal of bending (flexion and extension), the skin is liable to crack, as in cases of "cracked heels" and mud fever. This oil gives to the skin a polish, which naturally helps to check loss of bodily heat by radiation, and thus protects the body to some extent from chill. As a full supply of this oil is not secreted in many diseased conditions, the presence of the gloss which it gives to the coat is justly regarded as a valuable indication of good health. The more blood is brought to the skin by exercise and good grooming, the more plentiful will be the secretion of this oil.

5. To form epidermis, which is the general term for hair, cuticle, and horn, all of which are varieties of the scaly material (epithelium) formed by the true skin. Epidermis acts mechanically in protecting the body from injury. The short hair of the body of a horse is a protection against cold, and to some extent against irritating objects, such as flies. The duty of the long hairs of the tail, mane, and forelock is chiefly defensive, especially against insects. The long hairs of the muzzle serve almost entirely as feelers.

Dandruff consists of loose scales of the outer skin (cuticle), and of oil secreted by the skin. The cuticle and its dandruff* act like hair in protecting the skin against cold and against irritation from external objects. Dandruff, on the other hand, forms a comfortable nest for insects.