474 DIGESTION IN BIRDS. [BOOK II. 



provided with a widened portion, or with a caecal diverticulum, 

 which looks like an appendage. Portions of this kind, which are 

 characterised by modifications of the glandular organs of the mucous 

 membrane, form a crop (ingluvies). This is best developed in carni- 

 vorous and graminivorous birds ; in the former, indeed, it generally 

 forms a spindle-shaped enlargement, while in the latter it forms a 

 unilateral diverticulum, which is differentiated into a caBcal appendage, 

 in many provided with a narrow connecting piece." 



" The next portion of the oesophagus, which is generally narrower, 

 passes into the stomach, in which two divisions can be made out ; 

 the first is known as the proventriculus 1 ; its walls are generally thick- 

 ened by a glandular layer. The second is characterised by the great 

 development of its muscular layer, the strength of which varies with 

 the mode of life of the animal. Where it is greatly developed we may 

 observe a tendinous disc on either side. In the Raptores, as also in 

 many Natatores that live on animal food, the muscular layer is feebly 

 developed. It is very strong in the graminivorous forms (Gallinse, 

 Anatinoe, Columbse, Passeres). This portion, which serves for the 

 comminution of food, and compensates for the absence of masticatory 

 organs, may be provided with other arrangements also which serve the 

 same purpose ; its inner surface may be covered by a firm horny layer, 

 which is often of considerable thickness, and functions as a radula 2 ." 



Full of interest as are the processes of digestion in birds, our 

 knowledge of them is derived almost entirely from the writings of 

 the great naturalists of the past, and is in very few particulars 

 dependent on the researches of the moderns. It is to Borelli, Redi, 

 Spallanzani, Re'aumur and John Hunter that we are mainly indebted 

 for it. 



The function This diverticulum of the oesophagus exists in its 

 of the 'ingiu- most perfectly-developed form in graminivorous birds, 

 vies,' or 'crop.' The grain and other food which these birds swallow 

 first enters the crop, which becomes distended with it, and which 

 secretes a fluid which moistens its contents. According to Tiedemann 

 and Gmelin, the contents of the crop often present an acid reaction. 



So far as the Author is aware, no modern investigation has been 

 made with the object of determining the precise chemical characters 

 of the liquid secretion of the crop, and specifically to ascertain 

 whether it contains any digestive enzymes. The sojourn of food in 

 it is a long one, though it varies within wide limits. Tiedemann and 

 Gmelin noticed that grains swallowed by a hen remained in the 

 crop as long as twelve or thirteen hours, and Colin found oats in the 

 crop of a turkey eighteen and twenty hours after they had been swal- 

 lowed. According to the latter author, a fowl weighing 500 or 600 

 grms. requires from four to six hours before 10 grms. of grains have 

 passed out of the crop. 



1 Designated often 'ventriculus succenturiatus. ' 



2 Gegenbaur, Comparative Anatomy, pp. 557 and 558. 



