ORGANS OF DIGESTION. 403 



duce their food, like the gastric teeth of Crustacea, insects, 

 many gasteropods, and other invertebrata ; but in the carni- 

 vorous birds, with a thin membranous gizzard, no pebbles 

 are swallowed or required, the activity of the secretions effect- 

 ing all the necessary changes in the conditions of the food, 

 aided by the high temperature, and the movements of the 

 canal. From the proximity of the cardiac and pyloric orifices 

 of the gizzard, it forms a sac open only above, and is not 

 provided with the pyloric sphincter muscle, so common in 

 other vertebrated classes. This free and wide pyloric orifice 

 allows the food, partially digested in the three gastric cavities, 

 and reduced by the muscular action of the gizzard, to pass 

 out, in small successive portions, to the commencement of 

 the duodenum. The small internal capacity of this strong 

 grinding organ, which is chiefly filled with pebbles, necessi- 

 tates the development of other gastric cavities between it 

 and the mouth, to receive a sufficient quantity of coarse 

 vegetable food for the maintenance of these large and heavy 

 birds. 



The duodenum forms a long narrow duplication embracing 

 the bilobate conglomerate highly vascular pancreas (128. <?.y.)? 

 the tubuli of which, like those of the salivary glands, com- 

 mence with small vesicular enlargements, and terminate gene- 

 rally in two ducts which alternate with those from the liver 

 and gall-bladder. The small vitelline ccecum (129. A. g.) which 

 remains in several adult granivorous and wading birds, early 

 disappears in the rapacious tribes (129. B.), and still earlier 

 in the mammalia and in man. The two cceca-coli (128. s. s.) 

 are likewise of greatest size in the gallinaceous and other gra- 

 nivorous birds, where they arise, like those of the manis, by 

 two narrow canals (129. A, i.) from the commencement of 

 the short colon, and enlarge into wide sacs (129. A. /.), often 

 exceeding several times the size of the intestine, as in the 

 turkey. They are very large in many of the palmipeds, as 

 the swans, and they are connected by mesentery along the 

 sides of the small intestine. In the ostrich they have a spi- 

 ral fold of the mucous tunic extending through them, and 

 several folds of the same membrane are seen in the upper 

 part of the colon itself, which is here of unusual length. 

 They have no analogy to the urinary bladder, which is 

 below the rectal vestibule of birds, nor are they connected 



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