CHAPTER V. 



FREE SURFACES, EPITHELIUM, SECRETION, AND 

 INTEGUMENT. 



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 communi- 

 cating 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 con- 

 tinued over the faces of the articular cartilages. Synovial 

 bursce, provided for the free gliding of muscles or integument 

 over bone, such as the bursa on the front of the knee, the dis- 

 tention of which with fluid constitutes the common affection 

 called housemaid's knee, are similar sacs with more slender 

 walls; as also are the sheaths, called thecm, in which a 

 number of tendons glide. So also the interior of blood- 



