which carry away the perspiration from the body, are very fine, the end away from the surface being coiled up in such a way as to form a ball or oval-shaped body, constituting the perspiration gland.  The tube itself is also twisted like a corkscrew, and widens at its mouth.  It is estimated that there are between 2,000 and 3,000 of these perspiration tubes in every square inch of the skin.  Now, as we have already seen, the external skin of an ordinary adult is equal to an area of about twelve square feet, and in a tall person it may be as much as eighteen square feet.  The number of these tubes, therefore, in the whole body will be many hundreds of thousands, so that it will readily be seen how exceedingly important it is that they should be kept in thorough working order by cleanliness.  The two great purposes fulfilled by the perspiration are the removal by its means of worn-out or effete material which is injurious to the system, and the regulation of the heat of the body by its influence.  When it is stopped by any reason, such as catarrh or disease, the skin fails in its work, and the noxious matters, instead of being expelled from the body, are thrown back into the system.  Hence there is a good deal of truth in the belief that a freely acting skin is always a safeguard against disease.

The second variety of tubes, those which furnish an oily-like fluid to the skin, resemble in—­great part those which serve for the office of perspiration.  At the extremity away from the surface of the body, each one has a gland, the oil gland, which secretes the oily material.  The pores or outlets which open on the skin, however, are a good deal larger than the similar orifices of the perspiratory tubes, but they are not distributed so equally throughout the body.  In certain parts of the skin they are especially numerous, as on the nose, head, ears, and back of the shoulders.  The unctuous matter which is secreted by these oil glands is intended to keep the skin moist and pliant, to prevent the too rapid evaporation of moisture from the surface, and to act as a lubricant where the folds of the skin are in contact with each other.  At times in these oil tubes the contents extend to the opening on its surface; the part in contact with the air then becomes darkened, and forms the little black spots so frequently seen on the face of some persons.  The white, greasy matter which is thus contained within the tubes can often be squeezed out with the fingers or a watch key, and on account of its shape and black end is popularly supposed to be a grub or maggot.

The tube into which each hair of the body is inserted differs materially from the two preceding, in that its function is more restricted.  It serves to form a sort of sheath which contains each hair, and is called the hair follicle.  Usually one of the last described ducts opens directly on the side of the hair follicle, and its secretion serves the purpose of keeping the hair pliant.  It will be more convenient, however, to enter into a fuller description of the hair and hair follicle when be come to speak of the hair, the nails, and the teeth.