DIGESTIVE SYSTEM 751 



Those coming from the liver are called hepatic ducts; those from the 

 gall bladder are called cystic ducts (Fig-. 436). These unite, forming 

 the common duct which is also called the choledochal duct. It is this 

 common or choledochal duct which empties into the intestine. The liver 

 has many and varying shapes in the different animals, depending to a 

 large extent not only upon the shape of the body but on the shape and 

 size of the organs which press upon it. The color of the gall may vary 

 from a brown, yellow, purple or green, to a vermilion. 



THE PANCREAS 



This is the second largest of the digestive glands (Fig. 436) and 

 secretes digestive ferments of great strength, such as trypsin, steapsin, 

 and amylopsin, which digest both proteins and carbohydrates. 



In some respects the pancreas resembles the salivary glands and so 

 compensates in part for the absence of such glands in the lower verte- 

 brates. This pancreas arises, as already mentioned, from the dorsal wall 

 of the intestine, close to the liver. There are usually three diverticula, 

 one dorsal and two ventral. These latter soon unite. In sharks there 

 is only a single diverticulum, while in the sturgeon there are not only 

 two dorsal but also an equal number of ventral. The proximal portion 

 forms the ducts, the distal, the glands. The number of ducts that per- 

 sists varies immensely. In some forms of animals all but one disap- 

 pears, while in the lampreys all may be lost. However, in the mammals 

 there are usually two ducts persisting, the ventral known as the pan- 

 creatic or Wirsung's duct, and the dorsal called the accessory or San- 

 torini's duct. Again, the ducts may all remain distinct, or they may 

 unite before they enter the intestines. One of them may even unite 

 with the bile duct. While not absolutely proved, it seems that all verte- 

 brates have some form of pancreas. This may be only a slender tube in 

 the mesentery, as in teleosts, or it may lie outside the muscles in the 

 intestinal walls as in dipnoi. In the cyclostomes it is partly concealed 

 at the insertion of the spiral valve, partly in the liver. In these forms, 

 however, the duct has entirely disappeared so that it forms one of the 

 ductless glands, or in other words a gland of internal secretion. The 

 pancreas varies in shape and size. It may be long and straight or pos- 

 sess many lobules. Almost always it is placed between the duodenum 

 and the stomach. There is a question as to whether or not the gland 

 is composed of two separate and distinct structures. 



SUMMARY OF THE DIGESTIVE SYSTEM 

 AMPHIOXUS 



The mouth lies at the bottom of a vestibule (Fig. 437) and is an 

 oral funnel bounded by ciliated buccal tentacles, with cartilaginous sup- 



