THE STOMACH 1151 



oesophagus. This is the gastr o-phrenic Hgament. A strong fold of the membrane 

 also extends from the diaphragm (opposite the tenth and eleventh ribs) to the 

 splenic flexure of the colon, and is known as the phreno-colic (costo-colic) ligament 

 [lig. phrenicolienale]. (See figs. 905, 906.) 



Minute anatomy. — The peritoneum, like all serous membranes, consists of two layers; a 

 lining layer composed of simple squamous epithelium (mesothelium), and an underlying 

 layer of fibrous connective tissue. The latter is highly elastic, and denser in the parietal than 

 in the visceral layer. It often contains fat. In mesenteries and similar structures, the con- 

 nective tissue is usually very scanty, except surrounding the vessels and nerves. Ruptures 

 often occur in the omenta, which thus become fenestrated in structure. The visceral peritoneum 

 is usually closely attached to the organs for which it forms the outer serous tunic, but the pa- 

 rietal peritoneum is often loosely attached to the adjacent wall by a fatty subserous layer [tela 

 subserosa]. Smooth muscle occurs frequently in the various peritoneal folds. 



The peritoneal cavity contains normally a very slight amount of watery fluid, which 

 serves to lubricate the smooth peritoneal surface and thus to eliminate friction between 

 adjacent surfaces during the movements of the ahmentary canal. 



Vessels and nerves. — The peritoneum is in general somewhat sparsely supplied with blood- 

 vessels from various adjacent trunks. Lymph-vessels also occur, but they probably do not 

 connect directly with the peritoneal cavity by stomata (as is found in the frog and as claimed by 

 some to occur in man). They communicate with the lymphatics of neighbouring regions. 

 The nerves are also comparatively scarce. They are partly of sympathetic origin (vasomotor), 

 and partly sensory nerves from the intercostal (7th to 12th), and lumbar nerves. The sensory 

 nerves are more frequent in the parietal peritoneum and end in the connective tissue, either 

 freely or in special end-organs (varying from simple end-bulbs to Pacinian corpuscles). 



Development. — -The principal features in the development of the peritoneum have already 

 been mentioned in the section on Morphogenesis and in the remarks on the general morphology 

 of the intestinal canal (p. 19). Further details will be included later under the development of 

 the intestine, etc. 



Variations. — Variations in the form and relations of the peritoneum are exceedingly 

 common, and are most commonly of developmental origin. Variations in the form and re- 

 lations of the various abdominal organs necessarily involve corresponding modifications in 

 the peritoneum. The diaphragm may be incompletely formed, leaving the peritoneal cavity 

 in communication with the pleural, or more rarely the pericardial cavity. The primitive 

 dorsal mesentery of the intestine [mesenterium commune] may persist unmodified (in about 

 2 per cent, of adults), or the various secondary changes may be inhibited at any stage. Thus 

 the'stomach or the intestinal loop may fail, either wholly or partly, to undergo their character- 

 istic rotations. The adhesions of the various mesenteries may be incomplete, or they may be 

 more extensive than usual. For example, the sigmoid mesocolon may be more or less com- 

 pletely obliterated by adhesion, and numerous unusual peritoneal pockets or ligamentous bands 

 may be formed in this way in various localities. Variations thus due to extensions of the normal 

 developmental process are sometimes difficult to distinguish from pathological adhesions 

 caused by peritonitis. 



Comparative. — As previously mentioned, the primitive body cavity in vertebrates extends 

 throughout the trunk region. In the cyclostomata, this primitive relation persists, the peri- 

 cardial cavity remaining in communication with the general body cavity. In aU higher forms, 

 however, the pericardial cavity becomes entirely separated. In amphibia the lungs He in the 

 general (pleuroperitoneal) body cavity; in the reptiles and birds, they are partially separated; 

 but a complete separation of the plein-al cavities occurs only with the formation of the definite 

 diaphragm in mammals. 



The formation in the peritoneal cavity of a complete dorsal mesentery, and an incomplete 

 ventral mesentery (in the hepatic region) is typical for all classes of vertebrates. Slight 

 modifications in the form of the mesenteries depend chiefly upon the different degrees of com- 

 plexity in the development of the various parts of the intestinal tract. The marked changes 

 associated with extensive secondary adhesions of the primitive peritoneal structures are found 

 only among the higher mammalia, especially in man. 



THE STOMACH 



The stomach [ventriculus; gaster] is a dilation of the alimentary canal suc- 

 ceeding the oesophagus. In the stomach the food is mixed with the gastric juice 

 and reduced to a viscid, pulpy liquid, the chyme [chymus], which undergoes a 

 certain amount of digestion and absorption before passing into the duodenum. 



The stomach (figs. 906, 907) is a somewhat pear-shaped organ located in the 

 upper, left side of the abdominal cavity. It presents a bodij [corpus ventriculi], 

 with an enlarged upper end or fundus, on the right side of which is the cardia, 

 the aperture communicating with the oesophagus. The body of the stomach is 

 extremely variable in form, as will be explained later, but is in general divisible 

 into a more expanded upper two-thirds, the cardiac -portion [pars cardiaca], 

 which is nearly vertical, and a more constricted lower third, the pyloric portion 

 [pars pylorica], which turns horizontally toward the right. The pyloric portion 

 often presents toward its lower end a slight, variable dilation, the antrum pylori, 



