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.