650. The oil-glands secrete an oil, partly free and diffused, and partly mixed with albumen. When the cells are fully formed, that is, fully distended, they yield their contents, and the fluid matter they contain is set free, and passes along the tubes to the surface; this fluid matter constitutes the oily element of the economy of the skin.
651. The uses of the unctuous product of the oil-glands are twofold: 1st. The protection; 2d. The removal of waste 298 matter from the system. In the exercise of these offices the oily substance is diffused over those parts of the skin which are naturally exposed to vicissitudes of temperature and moisture,—as the nose, face, and head;—to the injurious attrition of contiguous surfaces,—as the flexures of joints;—or the contact of acrid fluids,—as in the excoriations to which infants are liable.
Give the illustration. 648. Why is it necessary that the cutaneous nerves be kept in a healthy state? 649. Through what membrane does a large proportion of the waste material of the system pass? 650. What is the function of the oil-glands? 651. What are the uses of the oily product of these glands?
652. The oil of the unctuous substance is the principal agent in effecting these purposes: 1st. It prevents the evaporation or congelation of the water of the cuticle, which would cause it to become parched and peel off, thus leaving the sensitive skin exposed. 2d. It affords a soft medium to the contact of moving substances. 3d. It repels moisture and fluids. 4th. The action of these glands removes the waste atoms and purifies the blood.
653. In considering the purpose of the oily matter of the skin, there are two situations in which it deserves especial remark. 1st. Along the edges of the eyelids, where it is poured out in considerable quantity. Here, it is the means of confining the tears and moisture of the eyes within the lids, defending the skin from the irritation of that fluid, and preventing the adhesion of the lids, which is liable to occur upon slight inflammation. 2d. In the ears, where the unctuous wax not only preserves the membrane of the drum and the passage of the ear moist, but also, by its bitterness, prevents the intrusion of small insects.
654. The use of the perspiratory glands is to separate from the blood that portion of the waste matter which is carried off through the skin in the form of vapor. Sanctorius, a celebrated medical writer, daily, for thirty years, weighed 299 himself, his food, and excretions. He estimated that five of every eight pounds of food and drink passed from the system through the many outlets upon the skin. Many place the estimate much lower. All physiologists agree that from twenty to forty ounces of matter pass off from the skin of an adult every twenty-four hours.
652. What prevents the evaporation of the water of the cuticle? Give its 2d use. Its 3d. Its 4th. 653. What is said in reference to the distribution of the oily matter along the edges of the eyelids? In the ears? 654. Of what use are the perspiratory glands? How long did Sanctorius daily weigh his food, to ascertain the amount of secretion that passed through the skin?
655. The average amount of perspiration is about thirty ounces; and it passes off in such minute portions, and mixes so rapidly with the surrounding air, that it is not perceived. For this reason, it is called insensible perspiration. When this excretion is increased, it forms into drops, and is called sensible perspiration. The following experiments prove the existence of this excretion from the skin.
Experiments. 1st. Take a cold bell-glass, or any glass vessel large enough to admit the hand, and introduce it perfectly dry; at the same time close the mouth by winding a napkin about the wrist; in a short time, the insensible perspiration from the hand, will be seen deposited on the inside of the glass. At first, the deposit is in the form of mist; but, if the experiment be continued a sufficient time, it will collect in drops.
2d. Hold the apparently dry hand near a looking-glass, and the invisible vapor will soon be condensed, and cover the glass with a slight dew.