312 COMPARATIVE ANATOMY 



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

 mesenchymatous. The mesentery which attaches the stomach to the 

 dorsal body wall becomes the greater omentum of the adult, while the 

 ventral mesentery between the stomach and liver becomes the lesser 

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

 greater curvature of the stomach and results in its left-sided displacement. 

 The lesser curvature develops from the ventral side. In this way 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 epithelium 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. Furthermore, the innervation of the 

 stomach by a cranial nerve, the vagus, is regarded by some as a proof of the 

 derivation of the stomach from the anterior portion of the alimentary 

 canal. 



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 in Man 



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. 



Small Intestine. The small intestine extends, gradually diminishing 

 in diameter, from the pylorus to the ileocolic valve of the colon. The 

 small intestine is distinguished not only by its smaller diameter but also 



