PHYSIOLOGICAL ANATOMY OF THE SMALL INTESTINE. 259 



out easily only at the outer portions of the tube opposite the attachment of the mesen- 

 tery. Near the mesenteric border, the fibres are very faint. This is true throughout the 

 whole of the small intestine; although the fibres are most numerous in the duodenum. 

 The internal, circular, or transverse layer of fibres is considerably thicker than the longi- 

 tudinal layer. These fibres encircle the tube, running, for the most part, at right angles 

 to the external layer, but some of them having rather an oblique direction. The circu- 

 lar layer is thickest in the duodenum, diminishing gradually in thickness to the middle 

 of the jejunum, but after that maintaining a nearly uniform thickness throughout the 

 canal to the ileo-caecal valve. 



The jejunum, the second division of the small intestine, is continuous with the duo- 

 denum. It presents no well-marked line of separation from the third division, but is 

 generally considered to include the upper two-fifths of the small intestine, the lower 

 three-fifths being called the ileum. It has received the name jejunum from the fact 

 that it is almost always found empty after death. This portion of the intestine presents 

 no important peculiarities as regards its peritoneal and muscular coat. 



The ileum is somewhat narrower and thinner than the jejunum, otherwise possessing 

 no marked peculiarities except in the structure of its mucous membrane. This opens 

 into the commencement of the colon and is the termination of the small intestine. 



Mucous Membrane of the Small Intestine. The mucous coat of the small intestine is 

 somewhat thinner than the lining membrane of the stomach. It is thickest in the duo- 

 denum and gradually becomes thinner until we reach the ileum. It is highly vascular, 

 presenting, like the mucous membrane of the stomach, a great increase in the quantity 

 of blood during the process of digestion. It has a peculiar soft and velvety appearance, 

 and, during digestion, it is of a- vivid-red color, being pale-pink during the intervals. It 

 presents for anatomical description the following parts: 1, folds of the membrane, called 

 valvulaa conniveutes ; 2, duodenal racemose glands, or the glands of Brunner ; 3, intesti- 

 nal tubules, or follicles of Lieberkuhn ; 4, intestinal villi ; 5, solitary glands, or follicles ; 

 6, agminated glands, or patches of Peyer. 



The valvulaa conniventes, simple transverse duplicatures of the mucous membrane 

 of the intestine, are particularly well marked in man, although they are found in some 

 of the inferior animals belonging to the class of mammals, as the elephant and the camel. 

 They render the extent of the mucous membrane much greater than that of the other 

 coats of the intestine. Commencing at about the middle of the duodenum, they extend, 

 with no diminution in number, throughout the jejunum. In the ileum they become pro- 

 gressively more and more scanty, until they are lost at about its lower third. Sappey 

 .found about six hundred of these folds in the first half of the small intestine and from two 

 hundred to two hundred and fifty in the lower half. He estimates that, in those portions 

 of intestine where they are most abundant, they increase the length of the mucous mem- 

 brane to about double that of the tube itself; but in the ileum they do not increase the 

 length more than one-sixth. The folds are always transverse and occupy usually from 

 one-third to one-half of the circumference of the tube, although a few may extend entirely 

 around it. The greatest width of each fold is in the centre, where it measures from a 

 quarter to half an inch. From this the width gradually diminishes until the folds are 

 lost in the membrane as it is attached to the muscular coat. Between the folds are 

 found fibres of connective tissue similar to those which attach the membrane throughout 

 the whole of the alimentary tract. This, though loose, is constant, and it prevents the 

 folds from being effaced, even when the intestine is distended to its utmost. Between 

 the folds are also found blood-vessels, nerves, and lymphatics. 



The position and arrangement of the valvula3 conniventes is such that they move freely 

 in both directions and may be applied to the inner surface of the intestine either above 

 or below their line of attachment. It is evident that the food, as it passes along in obe- 

 dience to the peristaltic movements, must, by insinuating itself beneath the folds and 



