OESOPHAGUS, STOMACH, AND INTESTINE 341 



dermal portion of the cloaca (proctodseum), later becoming exca- 

 vated to form a vesicle. It is situated in the pelvic cavity between 

 the vertebral column and the posterior portion of the intestine, 

 opening into the outer section of the cloaca 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 unknown. 



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

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

 or less extent of striated fibres derived from those of the pharynx. 



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-life form, 

 the cardiac portion, into which the oesophagus opens, and the 

 fundus, which lies towards the left side of the abdomen being usually 

 more swollen and having thinner walls than the pyloric portion, 

 which communicates with the duodenum. The gastric glands 

 have in general a different histological and physiological character 

 in the three regions of the stomach, so that three glandular zones 

 may be distinguished (Fig. 251 ; and cf. p. 845) 



According to the definition given on p. 335, a true stomach is 

 wanting in Monotremes (Fig. 251, A); and although the organ is 

 represented by a wide sac, it is entirely wanting in glands, and 

 is lined throughout by stratified epithelium : this condition is 

 doubtless secondary. Amongst Edentates, a similar peculiarity is 

 seen in Manis javanica in which, however, some of the glands are 

 retained in a sac-like outgrowth from the greater curvature, the 

 rest of the stomach being lined by a horn-like layer. 



In herbivorous Mammals the stomach is, as a rule, relatively 

 larger and more complicated than in carnivorous Mammals, 

 and it may become divided into two or more chambers. In 

 Bradypus, many Rodents (Muridse) and in the Horse distinct 

 cardiac and pyloric chambers can be recognised, and in herbivorous 

 Marsupials and Ungulates numerous intermediate forms between 

 a simple and an exceedingly complex stomach, such as occurs in 

 the typical Ruminants, are to be met with (Fig. 251). In the 

 latter (Fig. 252) the stomach is divided into four chambers, which 

 are called respectively rumen (paunch), reticulum, psalterium, and 

 abomasum. The two first, which may be looked upon as parts of 

 one and the same chamber, simply serve as storage cavities, the 

 food returning from them into the mouth, once more to undergo 

 mastication. It then again passes down the gullet, and is con- 

 ducted along a groove to the psalterium, the edge of the groove 

 closing, and finally into the abomasum, the latter alone being pro- 

 vided with peptic (rennet) and pyloric glands, and serving as the 

 true digestive stomach : the other chambers are almost or entirely 

 glandless, and are lined by pavement epithelium. 



