1142 



DIGESTIVE SYSTEM 



cardium, etc. Pouch-like dilatations of the oesophagus may occur, especially in the upper 

 part of its posterior wall or at the lower end. According to C. R. Robinson, the latter include 

 (1) ampulla phrenica, just above the diaphragm, and (2) ayitrum cardiacum, in the abdominal 

 portion of the oesophagus. Diverticula also occur, some of which may be derived from the 

 embryonic vacuohzation of the epithehum previously described, as may likewise the occasional 

 congenital atresia. Abnormal strictures of the oesophagus may occur, oftenest at the upper 

 end, at the left bronchus, and near the lower end. Finally, the oesophagus may be in part 

 either double or absent, and may communicate by fistula with the trachea. 



Comparative. — The length of the oesophagus varies with the length of the neck, being 

 shortest in fishes and amphibia where the oesophagus is not well marked off from the stomach. 

 The lining epithelium is stratified squamous in mammals and birds, but often ciliated in lower 

 forms. Mucous glands are absent in fishes, but occur typically in all higher forms. They 

 are found best developed toward the lower end of the oesophagus, except in mammals, where 

 they are usually more numerous at the upper end. Dilatations may occur normally, as in 

 the crop of birds, which is richly supplied with glands. The musc^ilature of the oesophagus is 

 primitively entirely smooth (Oppel) as found in amphibia, reptiles and birds. A secondary 

 replacement by cross-striated muscle is found to a variable extent in the majority of mammals 

 and fishes. 



THE ABDOMEN 



The abdomen properly consists of that part of the body situated between the 

 thorax and the pelvis. It is bounded above by the diaphragm; below, by the brim 

 of the true pelvis; behind, by the vertebral column, diaphragm, quadrat us lum- 

 borum and psoas muscles, and by the posterior portions of the ilia. At the sides 

 it is limited bj^ the anterior parts of the ilia and the hinder segments of the muscles 

 which compose the anterior abdominal wall, viz., the transversus, internal oblique, 



. — Diagram of the Abdominal Regions. 



Joint between meso-sternum 

 and ensiform cartilage 



Tip of ensiform cartilage 

 Costal border 



■Upper horizontal plane 



Lower horizontal plane A, at 

 level of tubercles of iliac crest 

 Lower horizontal plane B, at 

 ' level of anterior iliac spines 



Vertical plane A, from middle 

 of inguinal ligament 



Vertical plane B, at lateral bor- 

 der of rectus (semilunar line) 



Summit of symphysis pubis 



In front, besides these muscles, there are the two recti and 

 External to the peritoneum the abdomen is lined by a 



and external oblique, 

 pyramidales muscles, 

 special layer of fascia. 



It is customary for anatomists and physicians to divide, for purposes of descrip- 

 tion, the ventral surface of the abdomen, })y means of two horizontal and two ver- 

 tical lines, into nine regions (fig. 898). A complete uniformity in the use of the 

 boundary lines marking these regional subdivisions has not as yet been attained, 

 although the variations in the schemes used are not marked as concerns the main 

 features. It should be borne in mind that it is necessary that the boundary lines 

 used should be converted into planes carried through the whole de])th of the abdo- 

 men and (h'fined on the dorsal as well as the ventral surface, and that the relations 

 defined can (;rily Ix; approximate, owing to the wide range of the physiological 

 variation in the position of the abdominal contents. The nine regions or subdivi- 

 sions may b(! outlintnl as follows: — The upper horizontal line or plane passes 

 through the lowest point of the tenth costal cartilages, about 5 cm. above the um- 

 bilicus, and dorsally through the second or third lumbar vertebra. The lower 



