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.]

The external surface passes into the interior of the body forming two surfaces, one of which, the intestinal canal, communicates in two places, at the mouth and anus, with the external surface; and the other, the genito-urinary surface, which communicates with the external surface at one place only.  The surface of the intestinal canal is much greater in extent than the surface on the exterior, and finds enormous extensions in the lungs and in the great glands such as the liver and pancreas, which communicate with it by means of their ducts.  The extent of surface within the lungs is estimated at ninety-eight square yards, which is due to the extensive infoldings of the surface [Fig 6], just as a large surface of thin cloth can, by folding, be compressed into a small space.  The intestinal canal from the mouth to the anus is thirty feet long, the circumference varies greatly, but an average circumference of three inches may safely be assumed, which would give between seven