THE DIGESTIVE SYSTEM 23 I 



The pyloric glands are limited to the pyloric portion of the stomach. 

 Their chief secretion is mucus, but the presence of some chief and parietal 

 cells suggests that they may also secrete some gastric juice. They differ 

 from gastric glands also in having relatively long pits and short, branched 

 and twisted tubules. Thus they resemble duodenal glands. 



Development of the Stomach. During ontogenesis, beginning with 

 the fifth week, the stomach arises as a local enlargement of the fore-gut. 

 Its lining, therefore, together with the glands derived from it, is endo- 

 dermal. The external peritoneal membrane is mesodermal ; the remainder 

 of the stomach wall, including the submucous and muscularis layers, is 

 mesenchymatous. The more rapid growth of the dorsal wall produces the 

 greater curvature of the stomach. The lesser curvature develops from 

 the ventral side. The original dorsal side shifts to the left side of the 

 body, while the primitive ventral side comes to lie towards the right. 

 Gastric glands begin to appear as local proliferations of the lining epi- 

 thelium during the seventh week. 



History of the Stomach. Since stomachs are not unknown among 

 invertebrates, it might be assumed that the stomach of vertebrates is 

 derived directly from that of invertebrates. However, among the proto- 

 chordates, the hemichordates and some urochordates possess a stomach, 

 while the cephalochordates do not, the pharynx passing immediately into 

 the intestine. The liver of Amphioxus develops as a ventral outgrowth 

 a short distance behind the pharynx. Consequently, if we consider 

 Amphioxus as an ancestral type, the stomach of vertebrates must have 

 arisen from the short portion of the alimentary canal which in cephalo- 

 chordates lies between pharynx and liver. The esophagus must likewise 

 have developed from this region. 



In the cyclostomes, the stomach is a slight enlargement of the ali- 

 mentary canal. As in the Dipnoi, there is no flexure. In most fishes, 

 however, the stomach becomes J-shaped by the bending of the pyloric 

 region, and this curvature persists throughout the vertebrate series. 

 The complications of stomachs such as are found in ruminants are of 

 considerable importance and interest. The stomach of the cow, for 

 example, is divided into four functional divisions, rumen, reticulum, 

 omasum (psalterium) , and abomasum. Since, however, such adaptations 

 to a special diet throw no light on the problem of human phylogenesis, 

 detailed description is omitted. 



The Intestine 



The intestine is the portion of the alimentary canal from the pylorus 

 to the anus. Its length averages about thirty feet, of which five feet 

 are included in the large intestine and the remainder in the small intestine. 



