(ESOPHAGUS, STOMACH, AND INTESTINE 



263 



graminivorous 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 

 trace the course of the phylogenetic differ- 

 entiation of the organ. 



The small intestine is usually of consider- 

 able length and becomes folded on itself to 

 a greater or less degree ; it varies, however, 

 both in form, length, and diameter. 



The straight large intestine opens into a 

 cloaca, and varies as to its relative diameter. 

 The caecum is usually paired, and may reach 

 a relatively enormous length (Lamellirostres, 

 Rasores, Ratitse). All kinds of intermediate 

 stages between this and an entire absence 

 of a caecum are to be met with. When 

 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 FIG. 211. -DIAGRAM or 

 with a spiral fold consisting of numerous 

 turns, as in the Ostrich. 



The so-called bursa Falricii is a structure 

 peculiar to Birds, and arises as a small, 

 solid, epithelial outgrowth from the ectoder- 

 mtil portion of the cloaca, later becoming 

 excavated to form a vesicle. It is situated 

 in the pelvic cavity between the vertebral 

 column and the posterior portion of the intestine, and 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 physio- 

 logical function is quite unknown. 



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

 marked off from the stomach, and its muscles consist to a greater 

 or less extent of striated fibres : in Ruminants the latter extend as 

 far as the stomach. 



The stomach 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 cardiac portion, into which the oesophagus opens, being usually 

 more swollen and having thinner walls than the pyloric portion 

 which communicates with the duodenum. 



According to the definition given on p. 257, a true stomach is 



THE (ESOPHAGUS 

 AND STOMACH OF A 

 BIRD. 



M, glan- 

 dular stomach; MM, 

 muscular stomach ; 

 MD, duodenum. 



