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 and eight square feet of surface, this being many times multiplied by adding the surfaces of the glands which are connected with it. A diagram of the microscopic structure of the intestinal wall shows how little appreciation of the extent of surface the examination with the naked eye gives [Fig. 7]. By means of the intestinal canal food or substances necessary to provide the energy which the living tissue transforms are introduced. This food is liquefied and so altered by the action of the various fluids formed in the glands of the intestine and poured out on the surface, that it can pass into the interior of the body and become available for the living cells. Various food residues representing either excess of material or material incapable of digestion remain in the intestine, and after undergoing various changes, putrefactive in character, pass from the anus as feces. [Illustration: FIG. 7.--A SECTION OF THE SMALL INTESTINE TO SHOW THE LARGE EXTENT OF SURFACE. (_a_) Internal surface. The small finger-like projections are the villi, and between these are small depressions forming tubular glands.] By means of the lungs, which represent a part of the surface, the oxygen of the air, which is indispensable for the life of the cells, is taken into the body and carbonic acid removed. The interchange of gases is effected by the blood, which, enclosed in innumerable, small, thin-walled tubes, almost covers the surface, and comes in contact with the air within the lungs, taking from it oxygen and giving to it carbonic acid.