234 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



liver, but it may be immersed in the substance of the gland. In some 

 cases, even in mammals, the gall bladder may be lacking. When a 

 gall bladder is present, three regions may be recognized in the bile 

 ducts. Those parts which lead from the liver to the connexion with 

 the bladder are called hepatic ducts ; these are met by the cystic duct 

 leading from the bladder, and the common duct, formed by the two and 

 which empties into the intestine is the choledochal duct (fig. 240). 

 The shape of the gland is in part determined by the shape of the body, 

 being long in elongate species, sometimes consisting of two consecutive 

 lobes. Another modifying factor is the shape and size of the adjacent 

 organs, stomach, etc. Usually the liver is divided into right and left 

 halves, these corresponding to the first division of the anlage, but these 

 halves are hardly indicated in some of the teleosts. Frequently, and 

 especially in mammals, the halves become subdivided into lobes of 

 varying size, which are arranged in various ways. The liver is rela- 

 tively larger in the ichthyopsida than in the amniotes, but the cyclo- 

 stomes have a small liver, that of the myxinoids being in two parts. It 

 is larger, too, in the flesh-eating than in the herbivorous species. The 

 blood supply, chiefly through the portal vein and to a less extent by the 

 hepatic artery (see circulation) is very large. The color of the gland 

 is very variable, especially in teleosts, where it may be brown, yellow, 

 purple, green and even vermilion. 



THE PANCREAS. 



The second largest of the digestive glands, the pancreas, secretes 

 digestive ferments of great strength (trypsin, steapsin, amylopsin), 

 which digest both proteids and carbohydrates. In some respects it 

 resembles the salivary glands and so compensates in part for the 

 absence of them in the lower vertebrates (p. 220). The pancreas 

 arises by diverticula from the wall of the intestine close to the liver. 

 There are usually three of these diverticula, one dorsal and two ventral, 

 the ventral soon uniting (fig. 242), but in the sharks there is only a 

 single dorsal, diverticulum, while in the sturgeon there are two dorsal and 

 two ventral. In a general way these develop much like the liver, the distal 

 portions of the divisions forming the glands, which are of the acinous 

 type; the proximal portions form the ducts. Of these ducts all may 

 persist; all but one may disappear, while in the lampreys all may be 

 lost. In many mammals two ducts persist, the ventral forming the 



