VI THE INTESTINES 255 



muscular tubing the mucous membrane is thrown into 

 folds which must evidently lie at right angles to its long 

 axis. These folds serve to increase the surface of the 

 mucous membrane and are called valvules conni- 

 ventes. 



The large intestine presents noteworthy peculiarities 

 in the arrangement of the longitudinal muscular fibres of 

 the colon into three bands, which are shorter than the 

 walls of the intestine itself, so that the latter is thrown 

 into puckers and pouches (Fig. 79, Q) ; these are known 

 as the sacculi, and serve for the same purpose as the 

 vdlvulcp conniveiites of the small intestine. Moreover, the 

 muscular fibres around the termiiiation of the rectum are 

 arranged so as to form a ring-like sphincter muscle, which 

 keeps the aperture firmly closed, except when defeeca- 

 tion takes place. 



The mucous membrane of both small and large intestine 

 consists mainly of a number of simple tubular glands 

 packed side by side ; they are known as the glands of 

 Lieberkiihri (Fig. 82, G.L). In the small intestine 

 the tissue between the mouths of the neighbouring 

 glands is at frequent intervals thrown out into the 

 cavity of the intestine as club-.shaped processes or 

 projections, the villi, which are thus set side by 

 side over the surface of the mucous membrane like the 

 pile on velvet. These villi are absent in the large intes- 

 tine. The glands of Lieberkuhn are separated from each 

 other by tissue which is practically the same as that 

 already described as lymphoid or adenoid tissue (p. 90) ; 

 it is therefore a network of connective tissue fibres. 

 Wherever a villus occurs this adenoid tissue is prolonged 

 up into and forms the body of the villus, and in it run the 

 finer branches of the blood-vessels and lymjihatics supj^lied 

 to each villus. Each gland of Lieberkiihn is lined by a 

 layer of cubical cells, among which occur a certain number 

 of mucous cells, and this layer is continued outwards 

 over each villus as its external covering ; but the cells 



