48 THE STORY OF THE BIRDS 



subjected to the mechanical process of tritura- 

 tion or crushing (especially in birds that subsist 

 upon grain and consequently possess muscular 

 stomachs) in the gizzard, the action usually 

 being facilitated by the presence of stones, grit, 

 or sand swallowed by the bird to aid digestion. 

 When thus prepared the food then passes on 

 through the pylorus into the small intestine, 

 where in the first loop of which (the duodenum) 

 it is mixed with the secretions of the liver and 

 pancreas, the action of which converts the 

 soluble portions into peptones which are con- 

 veyed into the lymphatic system and so into 

 the blood. When present and functional the 

 caeca receive what remains of the food, where 

 still more nutrition is extracted. The remaining 

 waste substance then returns to the rectum, and 

 is finally ejected through the cloaca or vent in 

 the form of faeces or " droppings." 



Passing over the organs of reproduction, we 

 come to those of respiration. Here, again, 

 space does not admit of much general descrip- 

 tion, although we must deal at some length with 

 the organs of voice, which are such a peculiarity 

 in birds. Leading to the organ of voice is the 

 Trachea or windpipe branching at the end into 

 the two Bronchi, each of which enters the lung 



