[Illustration: FIG. 3.--A SECTION OF THE SKIN. 1. A hair. Notice there is a deep depression of the surface to form a small bulb from which the hair grows. 2. The superficial or horny layer of the skin; the cells here are joined to form a dense, smooth, compact layer impervious to moisture. 3. The lower layer of cells. In this layer new cells are continually being formed to supply those which as thin scales are cast off from the surface. 4. Section of a small vein. 9. Section of an artery. 8. Section of a lymphatic. The magnification is too low to show the smaller blood vessels. 5. One of the glands alongside of the hair which furnishes an oily secretion. 6. A sweat gland. 7. The fat of the skin. Notice that hair, hair glands and sweat glands are continuous with the surface and represent a downward extension of this. All the tissue below 2 and 3 is the corium from which leather is made.] [Illustration: FIG. 4.--DIAGRAMMATIC SECTION OF A SURFACE SHOWING THE RELATION OF GLANDS TO THE SURFACE. (_a_) Simple or tubular gland, (_b_) compound or racemose gland.] All of the causes of disease act on the body from without, and it is important to understand the relations which the body of a highly developed organism such as man has with the world external to him. This relation is effected by means of the various surfaces of the body. On the outside is the skin [Fig. 3], which surface is many times increased by the existence of glands and such appendages to the skin as the hair and nails. A gland, however complicated its structure, is nothing more than an extension of the surface into the tissue beneath [Fig. 4]. In the course of embryonic development all glands are formed by an ingrowth of the surface. The cells which line the gland surface undergo a differentiation in structure which enables them to perform certain definite functions, to take up substances from the same source of supply and transform them. The largest gland on the external surface of the body is the mammary gland [Fig. 5] in which milk is produced; there are two million small, tubular glands, the sweat glands, which produce a watery fluid which serves the purpose of cooling the body by evaporation; there are glands at the openings of the hairs which produce a fatty secretion which lubricates the hair and prevents drying, and many others. [Illustration: FIG. 5.--A SECTION OF THE MAMMARY GLAND. (_a_) The ducts of the gland, by which the milk secreted by the cells which line all the small openings, is conveyed to the nipple. All these openings are continuous with the surface of the skin. On each side of the large ducts is a vein filled with blood corpuscles.] [Illustration: FIG. 6.--PHOTOGRAPH OF A SECTION OF THE LUNG OF A MOUSE. _x x_ are the air tubes or bronchi which communicate with all of the small spaces. On the walls of the partitions there is a close network of blood vessels which are separated from the air in the spaces by a thin membrane.]