38. The free surfaces to be noted in the study of the body are of three descriptions :

First, the external surface of the body is covered with integument.

Secondly, the surfaces of hollows and passages communicating freely with the outside, such as the alimentary canal and the ducts of glands, are lined with mucous membrane, so named from the mucus with which it is lubricated, thrown out from its surface, or supplied by glands.

Thirdly, the surfaces of cavities and canals which have little or no communication with the outside, have a smoothly polished appearance, and from them transudes, in the case of empty cavities, a sufficient amount of fluid to moisten them. This group of surfaces includes serous membranes, which are shut sacs of delicate membrane, extending over the surfaces of viscera, and lining the opposing walls of the cavities which contain them, so as to allow free gliding movement; thus the abdominal viscera and the opposed walls of the abdomen are invested with the peritoneum, the lungs and ribs with the right and left pleura, and the heart with the pericardium. The synovial membranes lining the cavities of joints are similar to serous membranes, but are not continued over the faces of the articular cartilages. Synovial bursae, provided for the free gliding of muscles or integument over bone, such as the bursa on the front of the knee, the distention of which with fluid constitutes the common affection called housemaids knee, are similar sacs with more slender walls; as also are the sheaths, called thecae, in which a number of tendons glide. So also the interior of bloodvessels and lymphatics are polished and smooth, and the interior of the heart is lined with membrane very similar to the pericardium round it.