2 PR 



A MANUAL OF ANATOMY 



In some places, however, only the outer longitudinal layer is present. 

 The mucous coat is beset all over with minute projections, called 

 villi, and is sometimes called the villous coat. These villi impart 

 to it a woolly appearance like the nap of velvet. It diminishes 

 in thickness from above downwards, and is characterized by the 

 following structures : (i) valvulae conniventes, (2) villi, (3) Brunner's 

 glands, (4) crypts or follicles of Lieberkiihn, (5) solitary glands, and 

 (6) agminated glands or Peyer's patches. 



Of the foregoing structures the valvulae conniventes, villi, and 

 Peyer's patches constitute the macroscopical (naked-eye) characters 

 of the mucous membrane, the others forming its microscopical 

 characters. 



The valvulae conniventes (valves of Kerkring) are permanent 

 folds of the mucous membrane, which cannot be effaced. They 

 are absent from the first part of the duodenum for a distance 

 of from I to 2 inches from the pylorus. Commencing about the . 

 upper end of the second part of the duodenum as small straggling 

 folds, they become large and distinct at the place of entrance of 

 the ductus communis choledochus and pancreatic duct (about 

 4 inches from the pylorus). Throughout 

 the rest of the duodenum and in the upper 

 part of the jejunum they are still prominent, 

 and are placed close to each other. In the 

 lower part of the jejunum they become 

 smaller, and are placed farther apart. In 

 the upper part of the ileum they become still 

 smaller and more irregular, and they finally 

 disappear just beyond the centre of the 

 ileum. They are crescentic folds placed 

 across the bowel, and each consists of two 

 layers of mucous membrane applied back to 

 back, with a little submucous areolar tissue 

 intervening. Their average length is about 

 2 J inches, and the average breadth of each. 

 is about J inch. The majority of them 

 extend round the bowel for from one-half to 

 two-thirds of its circumference. Some, how- 

 ever, describe complete circles, whilst a few 

 are arranged in a spiral manner so as to 

 describe from one to three turns round the 

 tube. Some of them begin and terminate 

 in bifurcated extremities, whilst others present abrupt single ex- 

 tremities. The purpose served by the valvulae conniventes is a 1 

 twofold one. In the first place they increase the extent of the ; 

 absorbing and secreting surface of the mucous membrane, and in ' 

 the second place they delay the passage of the intestinal contents, 1 

 and so afford time for digestion and absorption. i 



In connection with the valvulae conniventes of the duodenum 

 the common orifice of the ductus communis choledochus and : 



Fig. 337.— The Val 

 VVLM Conniventes, 



