THE ALIMENTARY CANAL 



363 



FIG. 339. SCHEMATIC DIAGRAM ILLUSTRATING PROBABLE RELATIONSHIP OF SYM- 

 PATHETIC NEURONS IN MYENTERIC AND SUBMUCOUS PLEXUSES. 



Motor neurons, stippled; sensory neurons, solid. 1, tunica propria; 2, muscularis 

 mucosse; 5, submucosa; 4, muscularis; M, myenteric plexus; S, submucous plexus; 

 a, axons; d, dendrons. (Kuntz, Jour. Comp. Neur., 23, 3, 1913.) 



SMALL INTESTINE 



The small intestine constitutes the longest portion of the digestive 

 tube. It connects the pylorus with the colon. It measures about 7% 

 meters (24 feet) in length. It may be divided into three segments: (1) 

 the duodenum., about 11 inches in length; (2) the jejunum, including 

 the upper two-fifth, about 9 feet, and (3) the ileum, including the 

 lower three-fifth, about 14 feet, of the remainder. The duodenum 

 lacks a mesentery, it is only partially enveloped by a serosa, and it has 

 the greater diameter, about 47 millimeters (2 inches). Below the 

 duodenum the caliber of the small intestine gradually decreases until 



