INTESTINAL DIGESTION. 309 



spinal column by a double fold of serous membrane, called 

 the mesentery. As the peritoneum which lines the cavity 

 of the abdomen passes from either side to the spinal col- 

 umn, it comes together in a double fold just in front of the 

 great vessels along the spine, and passing forward, splits 

 again into two layers, which become continuous with each 

 other and enclose the intestine, forming its external coat. 

 The width of the mesentery is usually from three to four 

 inches ; but at the commencement and the termination of 

 the small intestine, it suddenly becomes shorter, binding the 

 duodenum and that portion of the intestine which opens into 

 the eaput coli closely to the subjacent parts. The mesentery 

 thus keeps the intestine in place, but allows of a certain 

 amount of motion, so that the tube may become convoluted, 

 accommodating itself to the size and form of the abdominal 

 cavity. The form of these convolutions is irregular and is 

 continually changing. 



The length of the small intestine, in situ, is probably 

 from fifteen to eighteen feet ; 1 but the canal is very disten- 

 sible, and its dimensions are subject to constant variations. 

 When separated from the mesentery, and measured with- 

 out stretching, its length has been found to be, on an average, 

 about twenty feet. Its diameter is about one and a quarter 

 inches. 2 



This part of the alimentary canal has been divided into 

 three portions, which present anatomical and physiological 

 peculiarities, more or less marked. These are the duodenum, 

 the jejunum, and the ileum. 



The duodenum has received its name from the fact that 



1 See page 134, note. 



2 Dr. Brmton made the last-named estimates from measurements of something 

 less than forty human intestines. " In making these, the healthy intestine was 

 laid upon a board, and spread out to what seemed a proper width, before taking 

 its length. (Cyclopcedia of Anatomy and Physiology, London, 1859, vol. v., sup- 

 plement, p. 340.) Sappey, from measurements in four cases, estimates the 

 length of the small intestine at about twenty-six feet. ( Traite d'Anatomie Do 

 scriptivc, Paris, 1857, tome iii., p. 124.) 



