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 and eight square feet of
surface, this being many times multiplied by adding the surfaces of