ORIGIN AND CHARACTERISTICS 47 



the gizzard. This latter communicates with 

 the intestines (which vary considerably in 

 length in the various avine groups), which 

 tow^ards their extremity at the beginning of the 

 rectum in many birds are provided wdth a 

 pair of blind-sacs or lateral dilatations techni- 

 cally called caeca. The presence or absence of 

 these sacs in a functional condition is chiefly 

 dependent upon the nature of the food ; and 

 they are most developed in herbivorous species, 

 least so in those that subsist on animal food, 

 and absent in species that subsist on fruit and 

 grain. This, however, is a broad generalisation, 

 there are many exceptions ; and in some cases 

 one of these sacs only is present, and that in a 

 rudimentary state. The process of digestion 

 may be thus briefly summarised : after the 

 tongue has played whatever part is assigned 

 to it, the food is swallowed and passes through 

 the gullet assisted by moisture from numerous 

 glands into the stomach. In birds which have 

 a crop, however, the food is subjected to the 

 action of certain secretions w^hich, aided by the 

 heat of the body, exert a softening influence 

 and prepare it for its entry into the stomach. 

 Here again it comes under the influence of 

 secretions from the gastric glands, and is also 



