THE DIGESTIVE TUBE 247 



Johnson (in a paper about to be published) this bulb forms essentially 

 the zona columnaris in the anal part of the rectum. The anus is produced 

 after the cloaca has separated into dorsal and ventral portions. The 

 ventral division, which carries with it the allantois, becomes expanded to 

 form the bladder, but its outlet remains relatively narrow and becomes the 

 urethra. The outlet of the rectum is the anus, which is at first closed by 

 the anal membrane; this membrane ruptures in embryos measuring from 

 20 to 30 mm., except in the occasional cases of imperforate anus. The 

 tissue which subdivides the cloaca reaches the surface and constitutes the 

 perineum. 



In human embryos of about 10 mm. the intestinal loop becomes twisted 

 on itself (Fig. 241, D), and the large intestine is carried across the small 

 intestine in the duodenal region. The vermiform process thus comes to 

 lie on the right side of the body, and the colon, after it is withdrawn from 

 the umbilical cord into the body, is so bent as to form ascending, transverse, 

 and descending portions, below which, as the convoluted sigmoid colon, 

 it connects with the rectum. The disposition of the adult intestines de- 

 pends chiefly upon this primary torsion of the intestinal loop, and upon 

 the subsequent elongation 01 tne small intestine, which forms many loops 

 and coils. 



Meanwhile the yolk-sac has become detached, and its stalk has dis- 

 appeared, usually leaving no indication of its former position. The stalk 

 does not become the vermiform process, as was once supposed, but occa- 

 sionally it produces a blind pouch of the ileum, 3-9 cm. long, situated about 

 three feet above the beginning of the colon. This is the diverticulum ilei, 

 described and correctly interpreted by Meckel in 1812. 



The division of the intestine into six parts is a heritage from the Arabians. Duo- 

 denum, jejunum, ileum, caecum, colon and rectum were well recognized in the fifteenth 

 century, when, following Hippocrates, they were counted from below upward. The 

 various names which have been applied to them are discussed by Hyrtl (Das arabische 

 und hebraische in der Anatomic, Wien, 1879). Those which are now adopted have the 

 following significance. The rectum is the straight terminal portion. "Colon is the 

 KoiXov of Aristotle, which according to Pliny is a great source of pain (colic)." The 

 caecum, or blind intestine, was so named by Galen, who did not practice human dis- 

 section and so referred to the more elongated pouch in lower animals. The name has 

 generally been considered inappropriate for the human caecum. The Greek synonym 

 Tv<f>Xov (blind) is used in the medical term typhlitis (inflammation of the caecum). 

 The ileum (from aAe'co) is the coiled portion, and is arbitrarily defined as the lower 

 three-fifths of the small intestine. The jejunum (Lat., fasting) is the portion generally 

 found void and empty (Avicenna) , since food passes through it rapidly. The duode- 

 num, which has no free mesentery, was originally considered a part of the stomach; 

 its name indicates that its length is twelve finger-breadths. Hyrtl notes that the same 

 term has sometimes been applied to the rectum. 



Layers of the Digestive Tube. The wall of the digestive tube is com- 



