266 THE DIGESTIVE SYSTEM 



at the level of the seventh, eighth and ninth thoracic seg- 

 ments. The excitability of the cells in these segments is 

 temporarily increased, and this condition may manifest itself by 

 the presence of areas of cutaneous hyperalgesia (p. 195) in the 

 lower part of the right half of the epigastric region. Some of 

 the nerve-cells connected with the stomach (p. 249) are 

 situated in these segments and consequently, although the 

 gastric mucous membrane may be perfectly healthy, the 

 ingestion of food may give rise to referred pain, since the 

 afferent impulses become exaggerated as they pass through 

 the " focus of irritation." 



The bile duct also receives some of the terminal branches 

 of the vagus, and, on this account, a " focus of irritation " may 

 be established in the medulla oblongata following an attack of 

 biliary colic. It seems probable that some such condition is 

 responsible for the occurrence of vomiting at the end of an 

 attack of biliary colic or following the ingestion of food after 

 the cessation of the pain. 



It should be remembered that similar reflexes, though of a 

 less pronounced type, may accompany inflammation or other 

 pathological conditions of the gall-bladder or bile duct. Owing 

 to the propinquity of the gastric and the hepatic centres in 

 the spinal medulla, the symptoms produced by cholecystitis 

 may be misinterpreted and they may be erroneously ascribed 

 to some non-existent gastric disorder. For the same reason, 

 the symptoms produced by gastric disturbances may be errone- 

 ously ascribed to the gall-bladder, and the diagnosis may only 

 be corrected at a subsequent operation. 



THE PANCREAS 



The Pancreas is an elongated gland which lies obliquely 

 across the upper part of the posterior abdominal wall. With the 

 exception of its tail, which is situated between the two layers of 

 the lieno-renal ligament (p. 239), the pancreas is entirely retro- 

 peritoneal, and it is therefore practically fixed in position. 



