DIGESTIVE TUBE. 195 



remain as Meckel's diver ticulum. This is a blind pouch of intestine, 

 usually less than four inches long but sometimes much longer, found on 

 the small intestine some four feet from its termination. 



Anterior to the yolk stalk the entodermal tube forms successively the 

 pharynx, oesophagus, stomach, duodenum, and the greater part of the 

 small intestine; posterior to it, the remainder of the small intestine, the 

 large intestine and the rectum. The rectum terminates at the anus which 

 is formed as an ectodermal inpocketing closed in embryonic life by the anal 

 membrane. Rarely this membrane or the adjacent rectum remains im- 

 perforate at birth. A transient embryonic extension of the intestine be- 

 yond the anus toward the tail is known as "post-anal intestine." It early 

 disappears, and has not been drawn in Fig. 219. The stomach is a dilated 

 portion of the tube at first vertically placed in the median plane (C) but 

 later so turned that its left side is ventral (or anterior), as in D. The duo- 

 denum is a subdivision of the small intestine, the remainder of which is 

 arbitrarily divided into the jejunum (the anterior two fifths) and the ileum 

 (the posterior three fifths). Where the ileum joins the large intestine a 

 blind outpocketing of the latter occurs, consisting of the caecum and its 

 slender prolongation the vermiform process (processus vermiformis) . At 

 a certain stage (C) the intestines make a simple loop of which the large 

 intestine forms the posterior or lower limb. To produce the arrangement 

 characteristic of the adult, the loop becomes twisted, as in D, so that the 

 large intestine crosses the small intestine not far from the stomach; thus it 

 is possible for the large intestine nearly to encircle the small intestine which 

 becomes greatly convoluted, without, however, changing its fundamental 

 relations. Besides the vermiform process and caecum, the large intestine 

 includes the ascending, transverse, descending and sigmoid colon, the last 

 terminating at an arbitrary line at the rectum. The rectum proceeds to 

 the anus, but not straight as its name implies. 



The entoderm forms only the epithelial lining of the digestive tube 

 and that of its associated glands. (Besides innumerable accessory 

 glands these include the liver, pancreas, and the lungs.) Around the ento- 

 derm, the mesenchyma forms successively the following layers, the tunica 

 propria which contains the reticular tissue and lymph nodules, and the 

 muscularis mucosae, a thin layer of muscles. The epithelium, tunica 

 propria, and muscularis mucosae together constitute the mucous mem- 

 brane. It rests on the tela submucosa, a vascular connective tissue 

 layer containing the sympathetic plexus submucosus. The submucous 

 layer is followed by the tunica muscularis. This consists of two or more 

 layers of muscle fibers between which is the sympathetic plexus mycn- 

 tericus. Beyond the muscularis is the connective tissue tunica advent itia 



