THORAX AND ABDOMEN OF THE HORSE 105 



aditus^ (aditus ad bursam omentalem), over the lesser curvature of the 

 stomach, with the vestibule of the bursa (vestibulum burstv omentalis), 

 a smaller cavity included, roughly speaking, between the stomach and 

 the liver. The vestibule is bounded ventrally by the stomach and the 

 lesser omentum, and dorsally by the gas tro- pancreatic fold of peritoneum 

 and the tail of the pancreas. The gastro-pancreatic fold passes from 

 the saccus cascus of the stomach to the pancreas and duodenum, and is 

 attached in addition to the dorsal border of the liver and the caudal 

 vena cava. To the left the vestibule is bounded by the gastro -phrenic 

 ligament, a double fold of peritoneum passing from the stomach to the 

 diaphragm. 



It must not be supposed that the cavity of the omental bursa and 

 its vestibule is not part of the peritoneal cavity in general. If the 

 finger of one hand be passed towards the left along the surface of the 

 caudate process of the liver, and the other hand be introduced into the 

 vestibule of the bursa, the fingers of the two hands may be made to 

 meet at an opening, the epiploic foramen (of Winslow). The foramen 

 is small, being in the form of a slit not more than from 6 to 10 cm. 

 long. It lies a little to the right of the median plane, immediately 

 medial to the right kidney and dorsal to the porta of the liver. The 

 foramen is bounded dorsally by the caudate process of the liver and the 

 caudal vena cava ; and ventrally by the portal vein, the pancreas and 

 the gastro-pancreatic fold. 



Develoi^ment of the peritoneum. — The peculiarities of the omenta 

 and the omental bursa will perhaps be better understood if the develop- 

 ment of the peritoneum in this region is very brietiy described. 



In its early embryonic condition the alimentary canal is suspended 

 from the dorsal wall of the abdomen by a simple and continuous dorsal 

 mesentery, such as is a permanent feature in many reptiles. When 

 the cavities of the thorax and abdomen have been separated from each 

 other by the development of the diaphragm, a ventral mesentery (ventral 

 mesogastrium) can also be recognised. This forms a connection 

 between the stomach and duodenum and the liver, and between the 

 liver and the ventral wall of the abdomen. The liver, indeed, by its 

 development has caused the production of the ventral mesogastrium. 

 The lesser omentum of the adult represents that part of the ventral 

 mesogastrium that lay between the stomach and the liver ; while the 

 falciform ligament is the remains of the ventral mesogastrium between 

 the liver and the ventral wall of the abdomen. 



The term dorsal mesogastriurti is applied to that part of the dorsal 

 mesentery that is connected with the stomach. So long as the 

 ' Aditus [L.], a way or entrance, access. 



