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.

The genito-urinary surface is the smallest of the surfaces.  In the male (Fig. 8,—­27, 28, 30) this communicates with the general external surface by the small opening at the extremity of the penis, and in the female by the opening into the vagina.  In its entirety it consists in a surface of wide extent, comprising in the male the urethra, a long canal which opens into the bladder, and is continuous with ducts that lead into the genital glands or testicles.  The internal surface of the bladder is extended by means of two long tubes, the ureters, into the kidneys, and receives the fluid formed in these organs.  In the female (Fig 9) there is a shallow external orifice which is continued into the bladder by a short canal, the urethra, the remaining urinary surface being the same as in the male; the external opening also is extended into the short, wide tube of the vagina, which is continuous with the canal of the uterus.  This canal is continued on both sides into the Fallopian tubes or oviducts.  There is thus in the female a more complete separation of the urinary and the genital surfaces than in the male.  Practically all of the waste material of the body which results from cell activity and is passed from the cells into the fluid about them is brought by the blood to the kidneys, and removed by these from the blood, leaving the body as urine.