GIZZARD OF BIRDS. 



165 



and thick, and being of a horny nature, it is liable to be increased 

 by pressure and friction, and as it is most subject to these in- 

 fluences at the parts of the gizzard opposite the musculi lateraleS; 

 two callous buttons are there formed, ib. g, g. It is here that 

 the fibrous structure of the lining membrane can be most plainly 



84 



Gizzard of a Swan. vii". 



seen : and it is worthy of observation that the fibres are not 

 perpendicular to the j^lane of the muscles, but oblique, and in 

 opposite directions, on the two sides. Elsewhere the cuticular 

 lining is disposed in ridges and prominences, figs. 84, S6, h, which 

 vary in different birds, but are pretty constant in the same species. 

 In a Petrel {Procellaria glaclalis), tlie lining membrane is dis- 

 posed in a pavement of small square tubercles, like the gastric 

 teeth of some Mollusks. 



The cavity of the gizzard is so encroached upon by the grinding 

 apparatus, that it is necessarily very small, the two horny callo- 

 sities having their internal flat surfaces opposed to one another, 

 like ^millstones.' A crop is as essential an appendage to this 

 structure as is the ^ hopper ' to the mill ; it receives the food as it 

 is swallowed, and supplies it to the gizzard in small successive 

 quantities as it is wanted.^ 



Between the stomach of the carnivorous Eagle and that of the 

 graminivorous Swan there are numerous intermediate structures. 



' Thus we find in Parrots, where the gizzartl is remarkably small, that a crop is 

 present. A like receptacle exists also in the Flamingo, in which the gizzard is small 

 but stronjr. 



