AVES BIHDS. 



599 



Fig. 272. 



reservoir, is the existence of a crop, or dilatation of some part of the 

 gullet into a wide bag (tngluvict), wherein grain or other sub- 

 stances hastily picked up may be stored preparatory to digestion. 

 After expanding into the crop in those birds that possess this 

 cavity, the oesophagus again contracts to its former dimensions 

 (Jig. 21%, a) ; but just before terminating in the gizzard it again 

 dilates to form a second but smaller cavity (6), called the proven- 

 triculuS) or bulbus glandulosus^ in which the food undergoes further 

 preparation. The walls of the proventiculus are thickly studded 

 with large glandular follicles, variously disposed ; from whence a 

 copious secretion of " gastric juice? as it is called, is poured out 

 and mixed with the aliment. Having, therefore, undergone mace- 

 ration in the juices of the crop, and become subsequently saturated 

 with the gastric fluid, that constitutes so important an agent in 

 digestion, alimentary substances are at length received into the giz- 

 zard (c), where further preparation is necessary. 



(667.) The gizzard in such birds as feed upon vegetable substances 

 is an organ possessing immense strength ; and constitutes, in fact, a 

 crushing mill, wherein nutritive materials are bruised and triturated : 

 its cavity is very small, and lined 

 with a dense, coriaceous cuticu- 

 lar stratum ; and its substance is 

 almost entirely made up of two 

 dense and enormously powerful 

 masses of muscle, the fibres of 

 which radiate from two central 

 tendons (fig. 272, c), situated 

 upon the opposite sides of the 

 viscus. The action of these late- 

 ral muscles will obviously grind 

 and crush with great force what- 

 ever is placed in the central ca- 

 vity ; a process that is materially 

 expedited by the presence of hard 

 and angular pebbles, swallowed 

 for the purpose, by the assistance 

 of which the contained food is 

 speedily comminuted. 



Another and much feebler set of muscles (d) bounds the cavity 

 of the gizzard in the intervals between the great lateral masses, 

 which, receiving the food from the proventriculus, perpetually feed 



