558 COMPARATIVE ANATOMY. 



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 (A B pr) ; its walls are greatly 

 thickened by a glandular layer. The second portion 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 {A B). 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 (Gallmas, Anatinte, Columba?, 

 Pas seres). This portion, which serves for the comminution of food, 

 aud 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. It is 

 produced by a glandular layer, the secretion of which passes into 

 this firm stiff condition. 



In the Mammalia the fore-gut is more completely divided, owing 

 to the sharper delimitation of the oesophagus from the stomach, than 

 it is in almost any other division. In many cases the shape of the 

 stomach is of a low type. In the Phocidas it retains its position 

 parallel to the long axis of the body, while in other Mammals a 

 position transverse to this axis is the common one. 



We must regard a number of peculiarities, which sometimes 

 consist in an enlargement of the internal space, at others of a 

 differentiation of the primitively single, and, as we must suppose, 

 uniformly functional stomach, into several portions of different 

 function, as the results of adaptation to the material of nutrition. 



The first relation is implied by the transverse position of the 

 stomach, in consequence of which the great curvature gets to be 

 much tho larger, and, forming a swelling behind the cardiac portion, 

 gives rise to the fundus of the stomach. This is absent in most 

 Carnivora, but is developed in the Monotreinata, Marsupialia, Ro- 

 dentia, and Edentata, and is found also in most of the Primates. 



When the fundus is more largely developed the stomach may 

 be divided into several portions, but this division is not unfrequently 

 implied by the characters of the mucous membrane only (Equus). 

 This arrangement is carried farther by the development of a trans- 

 verse constriction ; thus, in many Rodents, the stomach is divided 

 into a cardiac and a pyloric portion, to which smaller diverticula 

 may be added on. Similar stomachs of a more complicated character 

 may be seen in many Marsupials (Halmaturus), and in the Cetacea. 

 The fundus is always a considerable enlargement, which, in the 

 Cetacea, is succeeded by a number of diverticula, which are attached 

 to the pyloric portion ; these give the stomach the appearance of 

 being made up of from four to seven spaces which communicate 

 with one another by connecting pieces of varying width. 



In the Ruminantia the complication is due to the share taken by 

 the oesophagus, the cardiac end of which bulges out on one side and 



