The Inner (Alimentary) Tube and Its Respiratory Derivatives 



::i 



anterior region of the embryonic intestine. In the adult it is more or less 

 displaced from its embryonic position ventral to the digestive tube, and 

 may be more or less definitely divided into lobes. The bile-duct is the 

 persisting connection of the liver to the intestine at the point of em- 

 bryonic outgrowth. 



The pancreas, much less bulky than the liver, develops by out- 

 growth from the wall of the embryonic intestine, usually close behind 

 the point of origin of the liver (Fig. 26). 

 There are commonly two outgrowths, one 

 dorsal and one ventral, and there may be as 

 many as four. There is much variation in 

 the number of connections persisting as 

 pancreatic ducts. 



As seen in the course of ordinary dis- 

 section, the liver and pancreas appear as 

 quite solid organs. They are, however, 

 essentially hollow. Beginning as tubular 

 outgrowths from the intestine, each of 

 these organs in its fully developed state 

 consists of a system of elaborately branched 

 tubules whose caliber is of small micro- 

 scopic dimension. In the case of the liver, 

 special technique and high magnification 

 are required to render the lumen of a 

 hepatic tubule even visible. In the ab- 

 sence of an obvious lumen, the hepatic 

 tubules are often misleadingly referred to 

 as "hepatic cords," but these "cords" are 

 penetrated by the excessively narrow bile- 

 capillaries which collect the products of 

 the secretory hepatic cells (Figs. 27, 28). 

 The bile-capillaries converge into larger 

 passages which, in turn, join to form the relatively large hepatic 

 ducts. The ducts from the several lobes join into the bile-duct. In 

 close relation to the bile-duct there is usually a storage receptacle, the 

 gallbladder. The liver is highly vascular, its chief source of blood 

 being a system of veins, the hepatic-portal system, draining blood 

 away from the stomach and intestine. This blood is especially rich in 

 the absorbed products of digestion. 



The pancreatic cavities are wide in contrast to bile-capillaries. 

 The main pancreatic duct or ducts branch successively, passing ulti- 

 mately into tubules of microscopic width. Each ultimate tubule termi- 

 nates in an enlarged globular alveolus (or acinus) whose central re- 



Fig. 24. Pyloric ceca. 

 (Stomach above, intestine 

 below.) (Courtesy, Owen: 

 " Comparative Anatomy and 

 Physiology of Vertebrates," 

 London, Longmans, Green & 

 Co., Ltd.) 



