234 COMPARATIVE ANATOMY. 



in direct proportion to the consistency of the food. In gramini- 

 vorous Birds we find the strongest muscular layer and the thickest 

 horny lining, while in the series of insectivorous Birds, up to the 

 Birds of prey, this condition becomes gradually less marked, and 

 the division of labour is less noticeable. Thus in the series of 

 existing Birds we can see the line along which the differentiation 

 of the organ has taken place phylogenetically. 



Finally the dilatation of the gullet of Birds, which is known 

 as the crop (ingluvies), must be mentioned (Fig. 191, Ig). A 

 false crop serving only as a food reservoir, and entirely wanting 

 in glands, and a true crop, rich in glands, which have a chemical 

 function, can be distinguished. The former is seen, e.g., in many 

 Ducks, in the Cassowary, Haliseus, and Otis, and the latter in 

 'Rasores, Columbidse, and others. The small intestine is 

 usually of a considerable length, and becomes folded on itself to 

 a greater or less degree ; it varies, however, both in form, length, 

 and diameter. At about the middle of its extent there is a small 

 caecum-like structure, the remains of the vitello-intestinal duct, 

 or diverticulum caecum. 



The straight large intestine opens into a cloaca, and varies 

 as to, its relative diameter. The caecum is usually paired, and 

 may reach an enormous length (Lamellirostres, Rasores, 

 Ratitae). All kinds of intermediate stages between this and an 

 entire absence of a caecum are to be met with. 



In those cases where the caecum is largely developed, it has 

 an important relation to digestion, as an increase of surface of the 

 mucous membrane is thus effected ; this increase may even be 

 carried further by each caecum being provided with a spiral fold 

 consisting of numerous turns, as in the Ostrich. 



The so-called bursa Fabricii is a structure peculiar to Birds. 

 It arises as a small solid epithelial structure, which later becomes 

 excavated to form a vesicle, and lies freely in the pelvic cavity 

 between the vertebral column and the posterior portion of the 

 intestine ; it extends to the outer section of the cloaca, into 

 which it opens, posteriorly to the urinogenital ducts. It is 

 probably present in all Birds, but becomes atrophied more or 

 less completely in the adult; its physiological function is quite 

 unknown. 



Mammals. The oesophagus, like that of Birds, is sharply 

 marked off from the stomach, and is differentiated at its proximal 

 end to form a pharynx, which is regulated by strong muscles. 



Under the influence of the food the stomach here undergoes 

 much more numerous modifications than are met with in any other 

 Vertebrate class. As a rule it takes a more or less transverse 

 position and has a sac-like form, the portion into which the oeso- 

 phagus opens being called the cardiac, and the part between 

 this and the duodenum the pyloric portion. 



