166 ANATOMY OF VERTEBRATES. 



but it is necessary to observe that the animal or vegetable nature 

 of the food cannot always be divined from the difterent degrees 

 of strength in the gizzard. Hardcoated coleopterous insects, 

 for example, require thicker parietes for their due comminution 

 than pulpy succulent fruits. 



In the subgenus Eaphones, among the Tanagers, the muscular 

 or pyloric division of the stomach is remarkably small and not 

 separated from the duodenum by a narrow pylorus. 



The parietes of the gizzard, like those of other muscular 

 cavities, become thickened when stimulated to contract on their 

 contents mth o;reater force than usual. In the Hunterian col- 

 lection this fact is well illustrated by preparations of the gizzard 

 of the Sea-Gull in the natural state, and that of another Sea-Gull 

 which had been brought to feed on barley. The digastric muscles 

 in the latter are more than double the thickness of those in the 

 Sea-Gull which had lived on fish.* 



The immediate agents in triturating the food are hard foreign 

 bodies, as sand, gravel, or pebbles. 



Pigeons carry gravel to their young. Gallinaceous Birds grow 

 lean if deprived of pebbles ; and no wonder, since experiment ^ 

 shows that unless the grains of corn are bruised, and deprived of 

 their vitality, the gastric juice will not act upon or dissolve them. 

 The observations and experiments of Hunter have completely 

 established the truth of Redi's opinion, that the pebbles perform 

 the vicarious office of grinding teeth. 



Hunter inferred from the form of hair-balls occasionally found 

 in the stomach of Cuckoos,^ that the action of the great lateral 

 muscles of the gizzard Avas rotatory. Harvey appears to have 

 first investigated, by means of the ear, as it were in anticipation 

 of the art of auscultation, the actions wdiich are going on in the 

 interior of an animal body, in reference to the motions of the 

 gizzard. He observes (^De Gencratioiie Animalium, in Opera 

 Omnia, 4to, p. 208), ' Falconibus, ac[uilis, aliisque avibus ex 



' XX-. vol. i. p. 149, prep. 522, d, and .523. 



2 Grains of barley, inclosed in strong perforated tubes, pass through the alimentary 

 canal unchanged. Dead meat, similarly introduced into the gizzard, is dissolved. 



^ The hairs of caterpillars devoured by this bird are sometimes pressed or stuck 

 into the horny lining of the gizzard, instead of being collected into a loose ball. They 

 are then neatly pressed down in a regular spiral direction, like the nap of a hat, and 

 have often been mistaken for the natural structure of the gizzard. One of these speci- 

 mens, exibited as such to the Zoological Society, was sent to me for examination, when, 

 upon placing scjme of the supposed gastric hairs under the microscope, they exhibited 

 the peculiar complex structure of the hairs of the larva of the Tigei'-moth (Arctia 

 caja), and the broken surface of the extremity which was stuck into the cuticular 

 lining was plainly discernible. See Proceedings of Zooi Soc. 1834, p. 9. 



