4 68 



THE BIRDS 



XVI. 21- 



the humming-birds, eating nectar, the beak being long or short accord- 

 ing to the type of flower visited, and the tongue provided with a 

 special tubular tip. 



i net 



22. Digestive system of 

 birds 



Once the food is in the 

 mouth it is manipulated by 

 the long, thin tongue, mois- 

 tened with saliva, which 

 usually consists of mucus 

 but is said to contain diasta- 

 tic enzyme in some seed- 

 eating finches. Food swal- 

 lowed down the oesophagus 

 may be stored in a large 

 receptacle, the crop, found 

 especially in grain-eating 

 birds; its lining is of oeso- 

 phageal structure (Fig. 281). 

 The true stomach is divided 

 into two parts, a glandular 

 proventriculus and a muscu- 

 lar gizzard. The structure of 

 the anterior chambers of the 

 gut varies greatly with the 

 diet. In grain-eating birds, 

 such as the pigeon, the crop 

 is large and the seeds are 

 first macerated by storage 

 there. They are then mixed 

 with peptic enzymes in the proventriculus and ground up in the mus- 

 cular gizzard, which in pigeons has a horny lining and also contains 

 numerous small stones. In insectivores and carnivores the crop is 

 usually smaller or absent, but is very large in some fish-eating birds. 

 In carnivores the gizzard has the character of a more normal stomach. 

 It was stated by John Hunter that herring gulls, normally living on 

 fish, readily take to eating grain, and that after a year or so of this 

 diet the gizzard becomes muscular and has horny walls. 



The peptic juice has powerful digestive powers and many carni- 

 vorous and fish-eating birds dissolve even the bones of their prey, 



b.d 



Fig. 281. Dissection of pigeon, 

 bile-ducts; cl. cloaca; coec. coeca; cr. crop; giz. 

 gizzard; int. intestine; k. kidney; /. liver; oes. oeso- 

 phagus; p. pancreas; p.d. pancreatic ducts; pr. pro- 

 ventriculus ; sp. spleen ; test, testis. (After Schimkewitsch 

 and Streseman.) 



