326 COMPARATIVE ANATOMY 



History of the Liver. The vertebrate liver has no homolog among 

 invertebrates, though many of these have organs which are called livers. 

 That the so-called liver or digestive gland of the urochordates is the 

 homolog of the vertebrate liver has not been demonstrated. The liver 

 of amphioxus is generally regarded as representing the beginning of that 

 of vertebrates. This is a ventral outpocketing of the intestine immedi- 

 ately behind the pharynx, which grows ventrally and forwards beneath 

 the pharynx, and remains a hollow sac throughout life. Its relations to 

 the blood vessels resemble those of the liver of vertebrates. 



The liver becomes bilobed in cyclostomes and elasmobranchs, and a 

 gall bladder is differentiated. In the higher vertebrates and man no 

 important morphological changes occur. The form, however, varies 

 with the shape of the abdominal cavity and the pressure of surrounding 

 organs. In snakes, for example, it becomes reduced again to a single lobe. 



The Pancreas 



The pancreas is a light pinkish organ about five inches in length, 

 extending across the abdominal cavity from a loop of the duodenum on 

 the right side to the left cohc flexure. Three regions are distinguished, a 

 head lying in the intestinal loop, a body, and a tail. In man, the pan- 

 creas usually has two functional ducts. One of these, the pancreatic or 

 Wirsung's duct, generally opens into the common bile duct; the other, the 

 accessory or Santorini's duct, opens into the duodenum about an inch 

 above the opening of the bile duct. 



The pancreas secretes tr3rpsinogen, which is converted into trypsin 

 through the action of enterokinase secreted by the intestinal glands. 

 Trypsin splits proteins into amino-acids. The enzyme amylopsin secreted 

 by the pancreas splits starch into monosaccharids. Another enzyme, 

 lipase or steapsin when activated by enterokinase breaks fats into fatty 

 acids and glycerine. Another enzyme, ereptose or erepsin, spHts pro- 

 teoses and peptones. The digestive activity of the pancreas is stimulated 

 through the endocrinal effect of secretions poured into the blood by the 

 intestinal glands when the acid chjmie enters the intestine from the 

 stomach. 



Besides this digestive function, the pancreas acting as an endocrine 

 gland regulates the sugar metabolism of the body by means of the endo- 

 crine insulin. 



The histological structure of the pancreas strikingly resembles that of 

 the parotid gland, both being compound acinous glands divided into lobes 

 and lobules by connective tissue septa which contain interlobular ducts, 

 blood vessels, and nerves. The acini of the pancreas, instead of being 

 hollow, contain central cells. The secretory pancreatic cells of the acini 

 are wedge-shaped in section and contain zymogen granules largely con- 



