xlii INTRODUCTION 



The first, which is superficially hardly distinguishable 

 from the gullet, is known as the "proventriculus." 

 Its walls are richly supplied with digestive glands. 

 From this first stomach the food passes into the sec- 

 ond, which, in birds such as Fowls and Pigeons, for 

 example, has extremely thick and muscular walls, 

 while its cavity is lined with a more or less dense skin, 

 thrown into a series of folds. A stomach of this kind 

 is known as a "gizzard"; but in birds which feed on 

 animal food, such as Hawks and Gulls, there is no 

 gizzard, the walls of this region of the stomach being 

 thin and soft. Where a true gizzard is developed, 

 sharp stones and sand are swallowed by the bird, and 

 these are stored in the gizzard to aid in the work of 

 grinding up the grain and other hard matter, which 

 form the bird's food. Even flesh-eating birds appear 

 to find it necessary to swallow indigestible matter with 

 their food, and this matter is furnished by the hair 

 and feathers of their victims. Digestion completed, 

 the indigestible residue becomes welded together into 

 a mass, and is ejected from the mouth in the form of 

 a "pellet." From the second stomach the food passes 

 on into the intestine, and here all the nourishing por- 

 tions are absorbed. 



The air-sacs are found in all birds. They take the 

 form of a number of thin-walled chambers lying on 

 the sides of the body, and in front of the merry- 

 thought. During life these chambers are filled with 

 air drawn from the lungs. They are really a part of 

 the lungs; but their precise structure need not be 

 described here. At one time it was thought these 

 great air-chambers served the purpose of decreasing 



