1172 



SPLANCHNOLOGY 



its posterior border or root, about 15 cm. long, is attached to the posterior abdominal 

 wall from the left side of the body of the second lumbar vertebra to the right sacro- 

 iliac articulation, crossing successively the horizontal part of the duodenum, the 

 aorta, the inferior vena cava, the ureter, and right Psoas muscle (Fig. 1040). Its 

 breadth between its vertebral and intestinal borders averages about 20 cm., and is 

 greater in the middle than at its upper and lower ends. According to Lockwood it 

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

 composed are contained bloodvessels, nerves, lacteals, and lymph glands, together 

 with a variable amount of fat. 





Central lacteal 



Smooth muscle fibers ** // 



Reticular tissue 



Columnar epithelium ' Tt 



FIQ. 1060. Transverse section of a villua, from the human intes- 

 tine, (v. Ebner.) X 350. a. Basement membrane, here some- 

 what shrunken away from the epithelium. 6. Lacteal, c. 

 Columnar epithelium, d. Its striated border, e. Goblet cells. 



FIG. 1059. Vertical section of a villus from the /. Leucocytes in epithelium. /'. Leucocytes below epithelium, 

 dog's small intestine. X 80. g. Bloodvessels, h. Muscle cells cut across. 



Meckel's Diverticulum (diverticulum ilei). This consists of a pouch which projects from the 

 lower part of the ileum in about 2 per cent, of subjects. Its average position is about 1 meter 

 above the colic valve, and its average length about 5 cm. Its caliber is generally similar to that 

 of the ileum, and its blind extremity may be free or may be connected with the abdominal wall 

 or with some other portion of the intestine by a fibrous band. It represents the remains of 

 the proximal part of the vitelline duct, the duct of communication between the yolk-sac and the 

 primitive digestive tube in early fetal life. 



Structure. The wall of the small intestine (Fig. 1058) is composed of four coats : serous, 

 muscular, areolar, and mucous. 



. The serous coat (tunica serosa) is derived from the peritoneum. The superior portion of the 

 duodenum is almost completely surrounded by this membrane near its pyloric end, but is only 

 covered in front at the other extremity; the descending portion is covered by it in front, except 

 where it is carried off by the transverse colon; and the inferior 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 and near the middle line by the superior mesenteric vessels. 

 The rest 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 (tunica muscularis) consists of two layers of unstriped fibers: an external, 

 longitudinal, and an internal, circular layer. The longitudinal fibers are thinly scattered over the 

 surface of the intestine, and are more distinct along its free border. The circular fibers form a 

 thick, uniform layer, and are composed of plain muscle cells of considerable length. The mus- 

 cular coat is thicker at the upper than at the lower part of the small intestine. 



The areolar or submucous coat (tela submucosa) connects together the mucous and muscular 

 layers. It consists of loose, filamentous areolar tissue containing bloodvessels, lymphatics, and 

 nerves. It is the strongest layer of the intestine. 



