THE JEJUNUM AND ILEUM 



1165 



THE JEJUNUM AND ILEUM 



The mesenteric portion of the small intestine is divided into an upper half 

 (or two-fifths) , the jejunum, and a lower half (or three-fifths) , the ileum. Although 

 the character of the gut changes considerably from the upper end of the jejunum 

 to the lower end of the ileum, the transition is gradual, and there is no definite 

 line of demarcation. In general, the jejunum is somewhat wider, has thicker walls, 

 is more vascular and has a more complicated mucosa. The lymphoid organs 

 (Peyer's patches) are, however, characteristic of the ileum. 



The jejunum begins at the duodeno-jejunal flexure. The first coil is variable 

 in direction, being found (in order of frequency) as follows: (1) downward, for- 

 ward and to the left; (2) directly forward and downward; (3) to the left, then 

 downward; (4) forward and to the right (Harman). Some further details as to 

 the position of the various succeeding coils are given later under the development 

 of the intestine (figs. 930, 931). While there is considerable individual variation, 

 it is true in general that the coils of jejunum occupy the upper and left portion of 

 the body cavity, while those of the ileum occupy the lower and right side, the lower 

 portion lying in the pelvic cavity. The ileum finally passes upward over the pel- 

 vic brim to the right iliac fossa where it terminates in the ileo-caecal orifice. 



Fig. 924. — Portion of the Small Intestine, Laid open to Show the Plic.e Circu- 



LARES. (Brinton.) 



The mesentery [mesenterium] is a fan-shaped fold extending from the duodeno- 

 jejunal flexure to the ileo-caecal junction. It is composed of a double layer of 

 peritoneum which encloses and supports the jejunum and ileum and their vessels, 

 connecting them with the abdominal wall. The root of the mesentery [radix 

 mesenterii] or parietal attachment, is only about 15 cm. long, corresponding to 

 a line extending from the duodeno-jejunal flexure obliquely downward and to the 

 right, across the transverse duodenum, the great vessels and the vertebral column 

 to the ileo-csecal junction (fig. 905). 



The visceral attachment of the mesentery to the intestine, corresponding to the length of 

 the jejuno-ileum, is nearly 7 metres long, and is thinner than at the root. The width of«the 

 mesentery, measured from parietal to visceral attachment, varies somewhat in different parts 

 of the canal, the average being 18 or 20 cm. (ranging from 15 to 22.5 cm.). It is narrow above 

 (also at the lower end), but reaches its full width about 30 cm. below its upper end. Between 

 the two peritoneal layers of the mesentery is a third layer [lamina mesenterii propria] con- 

 taining the superior mesenteric vessels (arteries, veins and lymphatics) with their branches and 

 accompanying nerves, the small mesenteric lymph-nodes (50 to 100 in number), and a variable 

 amount of fibro-adipose connective tissue. 



Minute anatomy. — The small intestine has the four typical layers, — mucosa, submucosa, 

 muscularis and serosa (figs. 927, 928). They are, in general, somewhat similar in structure 

 to those of the stomach (fig. 910), excepting the mucosa. 



The mucosa is lined with a simple cylindrical epithelium, underneath which is a fibrous 

 lamina propria, limited externally by a muscularis mucosce, as in the stomach. The muscularis 

 mucosse sends slender muscular bundles upward into the villi. The inner surface of the mucosa 

 (fig. 924) presents numerous coarse, closely set, transverse folds [plicae circulares]. These are 

 permanent, crescentic folds, involving both mucosa and submucosa, and usually extending one- 

 half to tw^o-thirds of the way around the lumen. They often branch and anastomose, sometimes 

 forming circles or spirals. The largest exceed 5 cm. in length and 3 mm. in width. The phcae 



