THE VALVULJE CONNIVENTES. 305 



fibres of tlie muscular coat of the small intestine are 

 arranged in two layers; those of the outer layer being 

 disposed longitudinally; those of the inner layer trans- 

 versely, or in portions of circles encompassing the canal. 

 They are composed of the unstriped kind of muscular fibre. 



Between the mucous and muscular coats, there is a 

 layer of submucous tissue, in which numerous blood-vessels 

 and a rich plexus of nerves and ganglia are imbedded 

 (Meissner). 



The mucous membrane is the most important coat in 

 relation to the function of digestion, and the following 

 structures which enter into its composition may be now 

 successively described ; the valvulce conniventes ; the villi ; 

 and the glands. The mucous membrane throughout is 

 lined on its inner surface by columnar epithelium. 



The valvulae conniventes commence in the duodenum, 

 about one or two inches beyond the pylorus, and becoming 

 larger and more numerous immediately beyond the en- 

 trance of the bile-duct, continue thickly arranged and well 

 developed throughout the jejunum ; then, gradually 

 diminishing in size and number, they cease near the 

 middle of the ileum. In structure they are formed by a 

 doubling inwards of the mucous membrane, the crescentic, 

 nearly circular, folds thus formed being arranged trans- 

 versely with regard to the axis of the intestine, and each 

 individual fold seldom extending around more than \ or * 

 of the bowel's circumference. Unlike the rugae in the 

 stomach, they do not disappear on distension. Only an 

 imperfect notion of their natural position and function can 

 be obtained by looking at them after the intestine has been 

 laid open in the usual manner. To understand them 

 aright, a piece of gut should be distended either with air 

 or alcohol, and not opened until the tissues have become 

 hardened. On then making a section, it may be seen that 



x 



