io6 THE BIOLOGY OF BIRDS 



gall-bladder which stores the bile secreted by the liver. 

 The gall-bladder is absent in many pigeons and parrots. 

 In most cases the liver is a two-lobed organ, and the apex 

 of the heart fits in between the two lobes. 



As to the uses of the liver, one may say briefly (i) that 

 it serves as a great intermediating sponge bet^^een the 

 digested sugars and proteins, brought to it by the portal 

 system, and that by keeping back certain things, e.g. surplus 

 sugar, it maintains a certain constancy of composition in 

 the blood, which would otherwise be apt to change after 

 each variation in meals. (2) Surplus sugar is stored in 

 the form of animal starch or glycogen— a reserve food for 

 the muscles. (3) The bile is largely of the nature of a 

 waste-product, but it is of some slight service in connection 

 with digestion. (4) The liver also begins the ehmination 

 of nitrogenous waste-products from the blood— the work 

 which the kidneys complete. 



In reference to the presence or absence of a gall-bladder, 

 it has been shown by Richard E. Scammon (191 6) that in 

 the pigeon the gall-bladder is developed to all appearance 

 in a perfectly normal way. Later on, in the majority of 

 cases at least, it is completely lost, though its duct persists 

 and grows to some size in further embryonic development. 

 There is a larger anterior hepatic duct which is not associated 

 with the gall-bladder or its duct at all, but conveys the bile 

 independently from the liver. There are other animals 

 besides ordinary pigeons in which the gall-bladder is 

 absent, such as lamprey and rat, but the reason for its 

 disappearance or absence is unknown. Even in man it is 

 occasionally absent. Scammon calls attention to the large 

 size of the bile-ducts in the embryos of both pigeon and rat, 

 and suggests that this may be a compensation for the absence 

 of the usual bile reservoir in the form of a definite sac. 



The coiled part of the intestine following the U-shaped 

 curve of the duodenum is known as the ileum, and it 

 continues the twofold work of digestion and absorption. 

 Digestion in birds is singularly perfect, as may be inferred 

 from the extreme shortness of the large intestine or rectum. 



