318 



AVES. 



mission, it passes at once to the stomach to be 

 there successively digested, and the gullet pre- 

 sents no partial dilatations to serve as a tem- 

 porary reservoir or macerating receptacle. But 

 in the larger Raptorial Birds, as the Eagles 

 and Vultures, which gorge themselves at un- 

 certain intervals from the carcasses of bulky 

 prey, the oesophagus does not preserve a uni- 

 form width, but undergoes a lateral dilatation 

 anterior to the furculum at the lower part of 

 the neck. This pouch is termed the ingluvies 

 or crop (b, Jig. 156). 



a 



Fig. 156. 



Digestive canal of an Eagle. 



In those birds, again, the food of which is 

 exclusively of the vegetable kind, as grains 

 and seeds, and of which consequently a great 

 quantity must be taken to produce the ade- 

 quate supply of nutriment, and where the 

 cavity of the gizzard is very much diminished 

 by the enormous thickness of its muscular 

 coat, the crop is more developed, and takes a 

 more important share in the digestive process. 

 Instead of a gradual cylindrical lateral dila- 

 tation of the gullet, it assumes the form of a 

 globular or oval receptacle appended to that 

 tube, and rests upon the elastic fascia which 

 connects the clavicles or two branches of the 

 furculum together. 



In the common Fowl the crop is of large 

 size and single (b,fig. 157 : I, fig- 171), but in 



Fig. 157. 



the Pigeon it is double, consisting of two lateral 

 oval cavities (b c,Jig. 158). 



The dilatation of the oesophagus to form 

 the crop is more gradual in the Ducks 

 than in the Gallinaceous birds. The crop is 

 wanting in the Swans and Geese. 



The disposition of the muscular fibres of the 

 crop is the same as in the oesophagus, but the 

 muciparous follicles of the lining membrane are 

 larger and more numerous. Tins difference is 

 most conspicuous in the ingluvies of the grani- 

 vorous birds, where it is not merely a temporary 

 reservoir, but in which the food is mixed with 

 the abundant secretion of the glands, and be- 

 comes softened and macerated, and prepared for 

 the triturating action of the gizzard and the sol- 

 vent power of the gastric secretion. 



The change which the food undergoes in 

 the crop is well known to bird-fanciers. If 

 a Pigeon be allowed to swallow a great quan- 

 tity of peas, they will swell to such an extent 

 as almost to suffocate it. 



The time during which the food remains 

 in the crop depends upon its nature. In a 

 common Fowl animal food will be detained 

 about eight hours, while half the quantity 

 of vegetable substances will remain from six- 

 teen to twenty hours, which is one among 

 many proofs of the greater facility with 

 which animal substances are digested. Mr. 

 Hunter made many interesting observations 

 on the crop of Pigeons, which takes on a 



Fig. 158. 



secreting function during the breeding season, 

 for the purpose of supplying the young pi- 

 geons in the callow state with a diet suitable 

 to their tender condition.* An abundant se- 

 cretion of a milky fluid of an ash-grey colour, 

 which coagulates with acids and forms curd, 

 is poured out into the crop and mixed with 



* Animal Economy, p. 235. 



