GASTKIC DIGESTION. 381 



When, however, the food comes in contact with this secretion, it has not 

 yet been crushed or comminuted ; consequently it is incapable of being 

 acted on by the gastric juice, which is powerless to digest cellulose mem- 

 branes. The contact of the food with the walls of the ventriculus leads 

 to the pouring out of a profuse acid secretion. Bathed with this fluid, 

 the food then enters the gizzard, where it is reduced by crushing to a 

 homogeneous pulp. The gizzard has thick, muscular walls, with a hard, 

 hornj' epithelium lining it, and is capable of exerting very great force. 

 Thus, it has been stated that iron tubes capable of supporting a weight 

 of five hundred and thirty -five pounds were completely flattened out 

 after passing through the gizzard of a turkey. This crushing is indis- 

 pensable for the digestion of grains, and is aided by the presence of 

 gravel, etc., almost always to be found in this organ. 



In carnivorous birds gastric digestion is simpler than in the her- 

 bivora. Such birds swallow their prey entire, if small enough to enter 

 the beak and O3sophagus ; if not, it is torn with the beak small enough 

 to be swallowed, and then the skin, hair, feathers, and all are carried to 

 the stomach with the flesh. As there is no crop in such birds, and the 

 ventriculus is but faintly distinguished from the gizzard, which is com- 

 paratively small, and whose walls have become thin and almost mem- 

 branous, we have, therefore, a simple process of gastric solution, since 

 the gizzard has lost its crushing power. The solvent power of the stom- 

 ach of carnivorous birds is very rapid and powerful, muscles, tendons, 

 and cartilages being rapidly dissolved. After about eighteen or twent}^- 

 four hours, bones and matters which have escaped digestion, or which 

 are insufficiently dissolved to pass into the intestine, are regurgitated 

 through the mouth, since the very narrow pylorus present in carnivorous 

 birds, as in carnivorous mammals, prevents the passage of everything 

 except fluids. 



There exists, again, a t}*pe of birds, midway between the purely 

 omnivorous and the granivorous, where the gizzard has but moderate 

 thickness and power. These birds also vomit indigestible substances. 



Birds have, in general, a very active digestion. Some may make as 

 many as twelve meals a day, in which they fill not only the stomach, but 

 also the gullet, pharynx, and beak, especially when feeding on soft sub- 

 stances like larvae or worms. Their appetite seems to return as soon as 

 there is the least place which can hold more food. The diet of birds 

 cannot be changed. The birds of prey, without gizzard or crop, cannot 

 feed on grains, although the gallinaceous birds maybe brought to accom- 

 modate themselves to an animal diet. 



Colin states that morsels of meat fed to sparrows appear in the giz- 

 zard in less than an hour, and reach the intestine within an hour and a 

 half; while debris of food may be found in the faeces in four or five hours. 



