THE LIVER. 133 



The appendix vermiformis is similar in structure to the 

 colon. The raucosa contains well- formed glands of Lieber- 

 kiihn, and it and the loose submucosa contain numerous lym- 

 phoid follicles and diifuse lymphoid tissue. The muscular 

 coat and peritoneal covering form the outer layers. 



The rectum has a mucous and submucous layer similar to 

 that of the large intestine. The covering of simple columnar 

 epithelium changes at the anus into the stratified squamous 

 variety. 



The internal circular muscular layer is thickened to form 

 the internal (involuntary) sphincter, over which the longitudinal 

 muscular layer spreads out to form a complete investment 

 (instead of consisting of three bands only). The external 

 sphincter is a thin voluntary muscle surrounding the anus. 

 The lower portion of the rectum has no peritoneal covering. 



The peritoneum is a large serous or lymph sac, formed by a 

 typical serous membrane covering the walls of the abdomen 

 (parietal layer) and the abdominal viscera (visceral layer). It 

 enables the latter to move freely. 



It is mainly made up of & fibrous membrane containing lym- 

 phatics and bloodvessels, lined on the free surface with poly- 

 gonal endothelium-cells, between which in places are lymph- 

 stomata. 



Beneath it is a variable amount of areolar subserous or sub- 

 peritoneal tissue, in which, especially in the parietal layers, 

 mesenteries, and omenta, masses of fatty tissue often accumu- 

 late. 



The blood, lymphatic, and nerve trunks which supply the 

 intestines are situated in the mesenteries, which also contain 

 numerous lymphatic glands. 



The Liver. 



The liver is usually classed as a gland, producing the bile 

 as a secretion (external). It is much more than this. 



All the venous blood from the stomach and intestines, con- 

 taining the products of digestion absorbed from the alimentary 

 canal, is collected in the portal vein, redistributed in capillaries 

 and passed through the liver as through a sieve, and is acted 



