THE KM ALL INTESTINE. 915 



The ileum, so called from the Greek word etAeiv (to twist), on account of its 

 numerous coils and convolutions, is narrow, its diameter being one inch and a 

 quarter, and its coats thinner and less vascular than those of the jejunum. It pos- 

 sesses but few valvulae conniventes, and they are small and disappear entirely 

 toward its lower end, but Peyer's patches are larger and more numerous. The 

 jejunum for the most part occupies the umbilical and left iliac regions, while the 

 ileum occupies chiefly the umbilical, hypogastric, right iliac, and pelvic regions, 

 and terminates in the right iliac fossa by opening into the inner side of the com- 

 mencement of the large intestine. The jejunum and ileum are attached to the 

 posterior abdominal wall by an extensive fold of peritoneum, the mesentery, which 

 allows the freest motion, so that each coil can accommodate itself to changes ir 

 form and position. The mesentery is fan-shaped ; its posterior border, about six 

 inches in length, is attached to the abdominal wall from the left side of the second 

 lumbar vertebra to the right iliac fossa (Fig. 490). Its length is about eight 

 inches from its commencement to its termination at the intestine, and it is rather 

 longer about its centre than at either end of the bowel. According to Lockwood, 

 it tends to increase in length as age advances. Between the two layers of which 

 it is composed are contained blood-vessels, nerves, lacteals, and lymphatic glands, 

 together with a variable amount of fat. 



Meckel's Diverticulum. Occasionally there may be found connected with the 

 lower part of the ileum, on an average of about three and a half feet from its ter- 

 mination, a blind diverticulum or tube, varying in length. It is attached to and 

 communicates with the lumen of the bowel by one extremity, and by the other is 

 unattached or may be connected with the abdominal wall or some other portion of 

 the intestine by a fibrous band. This is Meckel's diverticulum, and represents the 

 remains of the vitelline or omphalo-mesenteric duct, the duct of communication 

 between the umbilical vesicle and the alimentary canal in early foetal life. 



Structure. The wall of the small intestine is composed of four coats serous, 

 muscular, areolar, and mucous. 



The serous coat is derived from the peritoneum. The first or ascending 

 portion of the duodenum is almost completely surrounded by this membrane near 

 its pyloric end, but only in front at the other extremity ; the second or descending 

 portion is covered by it in front, except where it is carried off by the transverse 

 colon ; and the third or transverse portion lies behind the peritoneum, which 

 passes over it, without being closely incorporated with the other coats of this part 

 of the intestine, and is separated from it in the middle line by the superior mesen- 

 teric artery. The remaining portion of the small intestine is surrounded by the 

 peritoneum, excepting along its attached or mesenteric border ; here a space is 

 left for the vessels and nerves to pass to the gut. 



The muscular coat consists of two layers of fibres, an external or longitudinal, 

 and an internal or circular layer. The longitudinal fibres are thinly scattered 

 over the surface of the intestine, and are more distinct along its free border. 

 The circular fibres form a thick, uniform layer; they surround the cylinder of 

 the intestine in the greater part of its circumference, and are composed of plain 

 muscle-cells of considerable length. The muscular coat is thicker at the upper 

 than at the lower part of the small intestine. 



The areolar or submucous coat connects together the mucous and muscular 

 layers. It consists of loose, filamentous areolar tissue, which forms a nidus for 

 the subdivision of the nutrient vessels, previous to their distribution to the mucous 

 surface. 



The mucous membrane is thick and highly vascular at the upper part of the 

 small intestine, but somewhat paler and thinner below. It consists of the 

 following structures : next the areolar or submucous coat is a layer of unstriped 

 muscular fibres, the muscularis mucosce ; internal to this is a quantity of retiform 

 tissue, enclosing in its meshes lymph-corpuscles, and in which the blood-vessels 

 and nerves ramify. Lastly, a basement-membrane, supporting a single layer of 

 epithelial cells, which throughout the intestines are columnar in character. They 



