380 



THE DIGESTIVE SYSTEM 



by a reduplication of the mucous membrane, which is strengthened by 

 a thickening and overlapping of the circular muscular layers of both 

 small and large intestines. The valve itself consists of an upper and 

 lower segment enfolding the slit-like orifice; laterally the two folds unite 

 to form the frenulum, which encircles the colon and marks the boundary 

 between it and the cecum. 



The outer longitudinal muscular layer is continued directly from 

 the wall of the ileum to that of the cecum, and therefore pursues a 

 relatively shorter course than either the internal muscular layer or the 

 mucous membrane. Section of only the outer layer of the muscular coat 



FIG. 352. TRANSECTION OF THE VERMIFORM APPENDIX OF MAN. 



The submucosa contains much adipose tissue, and a number of large lymph nodules, 

 each with a dense periphery and a large, lighter, germinal center. Photo. X 10. 



permits one to straighten the fold of the intestinal wall and thus oblit- 

 erate the valve. In other words, the outer muscular layer ^is not included 

 in the valvular reduplication. 



The muscularis mucosce is slightly thickened at the margin of the 

 valve. At this point also, the villi become shorter and at the margin 

 of the cecal surface of the valve they entirely disappear. 



The following tabular statement of the more important characteris- 

 tics of the several portions of the alimentary tract may be of assistance to 

 the student in the identification of microscopic sections of these organs. 



