458 THE ALIMENTARY CANAL [CH. XXVII. 



the outside by some of the circular muscular fibres of the intestine, 

 which are contained between the outer surfaces of the two layers of 

 which each fold is composed. While the circular muscular fibres, 

 however, at the junction of the ileum with the coacum are contained 

 between the outer opposed surfaces of the folds of mucous membrane 

 which form the valve, the longitudinal muscular fibres and the peri- 

 toneum of the small and large intestine respectively are continuous 

 with each other, without dipping in to follow the circular fibres and 

 the mucous membrane. In this manner, therefore, the folding in- 

 wards of these two last-named structures is preserved, while on the 

 other hand, by dividing the longitudinal muscular fibres and the peri- 

 toneum, the valve can be made to disappear, just as the constrictions 

 between the sacculi of the large intestine can be made to disappear 

 by performing a similar operation. The mucous membrane of the 

 ileum is continuous with that of the caecum. That surface of each 

 fold of the ileo-csecal valve which looks towards the small intestine is 

 covered with villi, while that which looks to the caecum has none. 

 When the caecum is distended, the margins of the folds are stretched, 

 and thus are brought into firm apposition one with the other. 



