Organs of Nutrition 



129 



descends after it leaves the mouth. In some birds this 

 is a simple tube of the same diameter throughout, always 

 moist from the secretion of (mucous) glands which are 

 found abundant in its walls, but serving merely as a 

 passage for the food on its way to the stomach. 



In another class of birds an enlarged chamber is 

 present, called the crop. This serves a somewhat simi- 

 lar purpose as the external pouch of the pelican; that 

 is, it acts as a receptacle for food. No especial digestive 

 glands are found here, and the only agents acting on 



Fig. 102. — Brush, or tube-like tongue of Honey Creeper. Twice natural size. 



the food are water, the secretions of the salivary glands, 

 and the heat of the bird's body. The crop exists only 

 superficially in some birds, the dilation being hardly 

 noticeable. ' 



From these we find a succession of more distinctly 

 marked permanent crops, until in grain-eating birds this 

 organ is very prominent. If we examine an English 

 Sparrow after it has made a hearty meal in the chicken- 

 yard, we will find the crop filled with grains of wheat, 

 some cracked in pieces by the bill, others entire. 



