386 



RODENTIA. 



trees, or upon tender succulent plants. The 

 differences observable in the structure of the 

 stomach and intestinal canal correspond to 

 the variety of their food, and bear a relation 

 to the structure of their dental apparatus. 



Fig. 270. 



campagnols 

 (Georychus, 

 rangement. 



(Arvicola), and the lemmings 

 llinger), present a similar ar- 



Cheek pouches of the Canada Rat (Geomys 

 bursarius), 



Some genera, as, for example, the Canada 

 rat (fig. 270.) are remarkable for the pos- 

 session of capacious cheek pouches, in which 

 considerable quantities of food can be stored 

 up, and which, like the crop of birds, may be 

 considered as reservoirs, wherein nutriment 

 can oe retained preparatory to its introduction 

 into the stomach. 



The type of stomach most common in this 

 order is the following: the stomachal bag is 

 formed by two distinct pouches, which are 

 more or less separated from each other ; one 

 portion, situated to the left of the cardia, is 

 placed longitudinally, and is generally of a 

 cylindrical or conical shape. This portion is 

 frequently larger than the right portion ; it is 

 lined internally with a thick epidermis, which 

 terminates suddenly, and clearly indicates by 

 its margin the boundaries of this compart- 

 ment of the stomach. The right compartment, 

 which is situated more transversely and further 

 back, is of a conical shape, the apex of the cone 

 terminating at the pylorus. This second 

 portion has its walls thicker and more mus- 

 cular than the former ; its mucous membrane 

 is not lined with epidermis, but presents the 

 ordinary appearance. The distinction be- 

 tween these two portions is indicated ex- 

 ternally by a constriction. The oesophagus 

 enters the first compartment very near to the 

 point where it communicates with the second. 

 Such may be said to be the typical form of 

 the stomach in this extensive order, but many 

 families recede from it to a greater or less ex- 

 tent. 



In the squirrels (Sciurus) for example, the 

 stomach is not divided into separate cavities, 

 but is of a pyriform or oval shape, giving off 

 a conical or cylindrical portion, which ter- 

 minates in the pylorus. The first compart- 

 ment is lined internally with a thick epi- 

 dermis, which forms two oval lips, as it is 

 prolonged around the opening into the second 

 compartment, the lining membrane of which 

 is simply mucous, without any apparent epi- 

 dermic covering. The ondatras (Fiber, Cuv.), 



Stomach of the Water Vole (Arvicola amphibiui). 



g, oesophagus ; a,/, cardiac extremity of the stomach ; 

 c, its median constriction ; b, dilated pyloric extre- 

 mity of ditto ; e, pyloric pouch ; h, i, duodenum. 



In the Hudson's Bay lemming (Mus Hud- 

 sonius, Gm.), the shape of the stomach is 

 slightly different, it is situated transversely 

 and much elongated, without any division 

 into cavities ; the cardia opens at about the 

 upper third of its anterior border ; the left 

 cul-de-sac is cylindrical and of uniform size 

 with the pyloric portion, which is bent for- 

 ward and to the left side. 



The stomach likewise varies from the com- 

 mon type in the jerboa (Dipus, Gm.), 

 and in the leaping hares of the Cape 

 (Helamys). In the former it is globular, in 

 the latter pyriform and longitudinal, with a 

 large cardiac cul-de sac directed forwards, a 

 pyloric cul-de-sac, and a short cylindrical 

 pyloric portion, which is bent forwards. 



The rat moles (Spalax y Guldensledt) are 

 approximated to the lemmings and to the 

 campagnols in the shape of their stomach, 

 which is divided into two pouches, having the 

 oesophagus closely approximated to the 

 pylorus. 



In the muscardiri (M. avellenarius, L.) 

 the stomach offers a peculiarity in its struc- 

 ture, which distinguishes it not only from the 

 other species of this genus, but also from all 

 other Mammalia, the oesophagus imme- 

 diately beyond the diaphragm terminates in a 

 globular pouch, the walls of which are thick, 

 glandular, and exhibiting internally numerous 

 pores leading into crypts : this structure is 

 separated by a constriction from the stomach 

 properly so called. This latter organ forms 

 a large cul-de-sac of a slightly oval shape, 

 which gives off anteriorly, and to the right 

 side, a short bowel-like pyloric portion. In 

 this animal, therefore, there are two stomachs, 

 one of which corresponds with the glandular 

 stomach of birds, as will be seen further on. 

 The beaver exhibits traces of this structure. 



The stomach of the hamster (Cricctus, 

 Cuv.) approximates the common type de- 

 scribed above, the stomach being divided into 

 two pouches, separated by a deep constric- 

 tion ; the left pouch is cylindrical, the right 

 globular. The cardiac orifice is situated in 



