FOOD AND ITS UTILISATION 105 



torn from his very body — if not a * pound of flesh,' at least 

 enough to make a lunch basket," In this and in similar 

 cases the biologist has to link the extraordinary to the 

 ordinary, for in many birds there is an occasional moulting 

 of the lining of the gizzard. 



H. C. Curl (191 1) has studied the periodically ejected 

 lining of one of the hornbills {Hydrocorax hydrocorax), and 

 finds that it consists of a tough homogeneous secretion from 

 the glands of the stomach. It comes away along with a 

 certain amount of undigested refuse. 



The Intestine and its Annexes. — The stomach is 

 followed by the duodenum into which there open the two 

 or three ducts of the pancreas, or stomach sweetbread. 

 The presence of three ducts, as in the pigeon, is an index 

 to the fact that the gland has a threefold origin in the 

 embryo. For the pancreas, like the liver, arises as an out- 

 growth from the mid-gut (or mesenteron) ; it is, to begin 

 with, saccular — lined with endoderm and surrounded 

 externally with mesoderm. 



The use of the pancreatic juice is to continue the digestive 

 action of the stomach. The comminuted " chyme " from 

 the gizzard is acted on by three pancreatic ferments, one 

 changing starchy food into sugar, another changing protein 

 into peptone, and a third emulsifying the fat and changing 

 it into fatty acids and glycerol. In this way the chyme 

 becomes chyle. The absorption of the digested food is 

 greatly facilitated by the thousands of finger-like villi 

 projecting into the lumen of the gut, from the stomach to 

 the end. The sugars and the peptones pass into delicate 

 tributaries of the mesenteric veins which enter the villi ; 

 and these mesenteric veins combine in a hepatic portal 

 vein which breaks up in the liver. But the villi also contain 

 fine tributaries of the lymphatic vessels, and into these the 

 digested fat passes. The lymph vessels eventually open 

 into the venous system. 



The liver is usually the largest organ in the body ; it 

 communicates by two or three ducts with the duodenum 

 antl one of the ducts is typically connected with a 



