THE NERVOUS SYSTEM AND SPECIAL FUNCTIONS. 301 



When a bird breathes, it breathes all through its body, in 

 its many intermuscular spaces, and in many, if not most 

 of, its bones. In addition to these aerating chambers, 

 the pelicans and the cormorants have a remarkable system 

 of intercellular tubes just underneath the skin. This 

 remarkable pneumaticity makes it possible for the bird 

 to accomplish its wonderful flights, calling for huge car- 

 bon dioxid excretion, and immense oxygen supply. 

 From the lungs, there extend through the body of the 

 bird, chambers communicating with the intermuscular 

 cavities of the front thorax, the hind thorax, the wings, 

 the abdomen, the legs, and the subcutaneous regions, 

 when they are present, these forming the auxiliary 

 respiratory chambers. 



Digestive System. 



The modifications of the alimentary canal of birds 

 are somewhat similar to those of carnivorous and vegetable 

 feeders among the insects. The connection of the tract 

 with the blood circulation, and the manner and place of 

 oxygenation of the food current, more closely resembles 

 the arrangement in man, as birds have a definite circulat- 

 ory and respiratory system. 



The entrance is through the mouth, which enlarges 

 into the. gullet or esophagus. In many seed-eaters, this 

 gullet serves for the retention of large quantities of food 

 seeds until they can be digested. The seed-eaters are 

 among the most nervously organized of the birds, .use up 

 the most food, consume the most oxygen, and must eat 

 voraciously to build up after this tissue exhaustion. 

 Pelicans have the gullet enormously enlarged to serve in 

 prehension of their food as well as its retention. 



Below the gullet the canal is usually modified into a 



