MUCOUS MEMBRANES. 315 



into it. The larynx, trachea, bronchia, and air-cells of the lungs 

 are lined with mucous membrane. A similar mucous surface 

 may be traced from the opening of the urinary canal along the 

 urethra, bladder, and ureters, to their termination in the calyces 

 of the kidney ; also into the vasa deferenlia. In the female it 

 is prolonged from the vagina into the uterus and the Fallopian 

 tubes to their termination. 



Between these two great divisions of the internal integuments 

 no organic connection exists. Each may be viewed as a canal 

 of considerable extent, but presenting numerous contractions and 

 dilatations corresponding with those of the hollow organs which 

 they line. Their external surface is rough and flocculent, being 

 attached by cellular tissue to the contiguous textures. The firm- 

 ness of this attachment varies in different places. In the sto- 

 mach the mucous membrane is easily separated. From the py- 

 lorus to the ileo-csecal valve it gradually becomes more firmly 

 attached, and in the large intestines it adheres very closely to the 

 next coat below it till towards the extremity of the rectum, where 

 it is again loose. 



The thickness of this membrane is equally various. It is thick- 

 est in the stomach and duodenum, and thence diminishes gra- 

 dually towards the lower part of the small intestines. At the 

 ileo-csecal valve it increases somewhat, and in the large intestines 

 it is only about half as thick as in the stomach ; but it increases 

 towards the extremity of the rectum. Its firmness and power of 

 resistance is greatest in the stomach. 



The colour of the mucous membrane varies in different parts 

 of its extent. It is influenced also by the age of the individual, 

 and doubtless by the disease of which he died. When freed from 

 cellular tissue and mucus it is translucent and white, or grayish, 

 with a delicate rosy tinge. This tinge is owing to the blood- 

 vessels with which it is supplied. It deepens in the stomach 

 during the digestive process, doubtless because the quantity of 

 blood conveyed to it is then greatest. 



The plicae and valvula conniventes of this membrane are well- 

 known 'to anatomists. When viewed with a microscope it is 

 found covered with a vast number of minute downy processes, 

 giving it a flocky appearance. These have been called villi. 

 These villi are generally considered as ducts which secrete the 

 gastric juice when it is required for the purpose of digestion. 



