20 A HISTORY OF BIRDS 



thickened pads, which, by means of spirally arranged muscular 

 fibres, are made to rub together in opposite directions after the 

 fashion of a pair of millstones. But this work of trituration is 

 further aided by a mass of small stones swallowed, and retained 

 by the gizzard, for this purpose. In the pigeons of the genus 

 Ptilopus there are four such pads, so that the gizzard is cross- 

 shaped in section. In fish and flesh-eating, and in insectiv- 

 orous and fruit-eating birds, the gizzard is feebly developed, 

 the proventriculus and ventriculus passing insensibly the one 

 into the other. The latter, in fish-eating birds, such as the 

 Herons and Cormorants, may assume the form of a long oval 

 sac extending along the whole length of the abdominal cavity ; 

 while, by way of contrast, it should be mentioned that in other 

 piscivorous forms, such as the Tropic-birds, Pelicans and 

 Gannets, the gizzard or ventriculus is much reduced, the pro- 

 ventriculus being in consequence of considerable size. In the 

 Cassowaries and Emus and in Tanagers of the genus Euphones 

 similar relations obtain between these two portions of the 

 stomach ; but in the Hoatzin, which has also a greatly reduced 

 ventriculus, it is the crop which has become enlarged (p. 313). 



The inner lining of the stomach is naturally constantly 

 wearing away and being reproduced, but in some birds this 

 lining is suddenly cast off and ejected through the mouth as 

 in the Starling, Missel Thrush, Little Owl, Cuckoo and Horn- 

 bill. The Cuckoo and Trogons of the genus Harpactes are 

 further peculiar in that the inner lining of the gizzard becomes 

 beset with hairs, spirally arranged, derived from the caterpillars 

 on which these birds feed. 



In birds of prey the fur and feathers of the victim swallowed, 

 and in some grain-eating birds the husks, are formed into 

 " pellets " and ejected through the mouth. 



The intestines, with the external digestive glands — the 

 liver and pancreas — attached thereto demand little notice here. 

 Suffice it to say that three portions thereof may be distinguished 

 - — the duodenum^ a closed loop embracing the pancreas, and re- 

 ceiving the hepatic and gall-ducts ; the ilcmn or " small in- 

 testines," the longest portion of the gut, and the recttini. The 

 last-named is shut off from the ileum by a special valve, the 

 "ileo-csecal," which, while it permits of the contents of the gut 

 passing into the rectum, hinders any return thereof. 



