322 



AVES. 



Pigeons, however, are known to 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 rationa- 

 lity and truth of Iledi's opinion, that the peb- 

 bles perform the vicarious office of teeth. 



Hunter inferred from the form of hair-balls 

 occasionally found in the stomach of Cuckoos,t 

 that the action of the great lateral muscles of 

 the gizzard was rotatory. Harvey appears to 

 have first investigated, by means of the ear, as 

 it were in anticipation of the art of auscultation, 

 the actions which are going on in the interior of 

 an animal body, in reference to the motions of 

 the gizzard. He observes, (De Generatione 

 Animalium, in Opera Omnia, 4to. p. 208,) " Fal- 

 conibus, aquilis, aliisque avibus ex preda viven- 

 tibus, si aurem prope admoveris dum ventricu- 

 lus jejunus est, manifestos intus strepitus, 

 lapillorum illuc ingestorum, invicemque colli- 

 sorum percipias." And Hunter observes, 

 (Animal (Economy, p. 198,) " the extent of 

 motion in grindstones need not be the tenth of 

 an inch, if their motion is alternate and in con- 

 trary directions. But although the motion of 

 the gizzard is hardly visible, yet we may be 

 made very sensible of its action by putting the 

 ear to the sides of a fowl while it is grinding 

 its food, when the stones can be heard moving 

 upon one another." 



Tiedemann believes that the muscles of the 

 gizzard are in some degree voluntary, having 

 observed that when he placed his hand oppo- 

 site the gizzard, its motions suddenly stopped. 



The pyloric orifice of the gizzard is guarded by 

 a valve in many birds, especially in those which 

 swallow the largest stones. This valve in the 

 Ostrich is formed by a rising of the cuticle 

 divided into six or seven ridges, which close 

 the pylorus like a grating, and allow only stones 

 of small size to pass through. In the Touraco 

 the pylorus projects into the duodenum in a 

 tubular form. There is a double valve at the 

 pyloric orifice in the Gannet, and a single large 

 valvular ridge at the same part in the Gigantic 

 Crane. In this species and some other Waders, 

 as the Heron and Bittern ; also in the Pelecan, 

 and, according to Cuvier, in the Penguin and 



* Grains of barley, inclosed in strong perforated 

 tubes, pass through the alimentary canal unchanged. 

 Dead meat, similarly introduced into the gizzard, 

 is dissolved. 



t 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 specimens exhibited as such 

 to the Zoological Society was sent to me for exami- 

 nation, when, upon placing some of the supposed 

 gastric hairs under the microscope, they exhibited 

 the peculiar complex structure of the hairs of the 

 larva of the Tiger-moth ( Arctia Caja), and the 

 broken surface of the extremity which was stuck 

 into the cuticular lining was plainly discernible. 

 See Proceedings of Zool, Soc, 1834, p. 9. 



Grebe, there is a small but distinct cavity inter- 

 posed between the gizzard and intestine. An 

 analogous structure is found in the Crocodile. 



The intestines reach from the stomach to the 

 cloaca ; in relative length they are much shorter 

 than in the mammalia. In the Toucan, for 

 example, the whole intestinal canal scarcely 

 equals twice the length of the body, in- 

 cluding the bill. The canal is divided into 

 small and large intestines, sometimes by an 

 internal valve, sometimes by the insertion of a 

 single coecum, but most generally by those of two 

 coeca, which are always opposite to one another. 

 In a few instances there is no such distinction. 

 The small intestines and coeca are longest in 

 the vegetable feeders. The large intestine is, 

 with one or two exceptions, very short and 

 straight in all birds. 



The course of the small intestine varies 



somewhat in the different orders of Birds ; 



it is always characterized by the elongated fold 



or loop made by the duodenum, (fj\ Jig. 163,) 



Fig. 163. 



Abdominal viscera of a Pigeon. 



which fold receives the pancreas (q q) in its 

 concavity. 



In the Raptores the intestines are generally 

 disposed as follows : 



The duodenum forms a long and broad 

 fold, the lower part of which is commonly 

 bent or doubled upon itself: the intestine then 

 passes backwards on the right side of the ab- 

 domen, crosses to the left, and is disposed in 

 deep folds upon the edge of a scolloped mesen- 

 tery ; towards its termination the ileum passes 

 up behind the stomach and adheres to it, having 

 here but a narrow mesentery; then passing down 

 the posterior part of the abdomen the ileum 

 makes another loose fold and ends in the rec- 

 tum, which is continued straight to the cloaca.* 

 In the Owls the last fold of the ileum is nearly 

 as long as the duodenal fold, and the cceca 

 adhere to each side of the fold. 



In the Diurnal Raptores the intestinal canal 



* la fig. 156 the intestines are not represented 

 according to their natural arrangement. 



