1192 



DIGESTIVE SYSTEM 



Comparative. — The liver arises in all vertebrates as an outgrowth of the entodermic epi- 

 thelium of the intestine just beyond the stomach. In amphioxus it remains a simple saccular 

 diverticulum, but in aU higher forms becomes a compound tubular gland. The tubular char- 

 acter becomes masked, however (in amniota, and especially in mammals), by the abundant 

 anastomosis between the tubules, forming what is called a 'soHd' gland. The relations with 

 the portal venous system are constant. The liver frequently stores large quantities of fat, and 

 may even undergo a complete fatty metamorphosis (lamprey). The colour of the liver is usually 

 reddish-brown, but may be yellow, purple, green or even vermillion (due to bile pigments). 

 In size, the Uver is variable, but is usually relatively larger in anamniota. Among mammals, 

 there is great variation according to diet, the liver being relatively larger in carnivora, smaller 

 in herbivora, and intermediate in omnivora (including man). It is also relatively larger in small 

 animals (including young and foetal stages), probably on account of their more intense metabo- 

 lism. There are tjiaically two lobes, right and left, in the vertebrate liver. These are frequently 

 subdivided, however, especially in mammals, which often present numerous lobes. 



The gall-bladder is typically present, as in man, but varies in form, size and position. It 

 may be completely buried in the Uver. In some species it is absent, in which case the hepatic 

 ducts open directly into the duodenum by one or more apertures. The hepatic and cystic 

 ducts typically unite to form a common bile-duct, as in man, but there are numerous variations 

 in the detailed arrangement of the ducts. 



THE PANCREAS 



The pancreas (figs. 922, 954, 955, 956) is an elongated gland extending trans- 

 versely across the posterior abdominal wall behind the stomach from the duode- 

 num to the spleen. Through the pancreatic duct, opening into the descending 

 duodenum, flows its secretion [succus pancreaticus], which is of importance in 

 digestion. The pancreas also has a very important internal secretion. 



Fig. 954. — The Duodenum and Pancreas, Anterior View. 

 Superior layer of transverse meso-colon 



Descending part of 

 duodenum 



Duodeno-jejunal flexure 



Inferior layer of transverse meso-coloa 



Inferior part of duodenum 



Superior mesenteric vessels 



The pancreas is greyish-pink in colour; average length {in situ), 12 cm. to 15 

 cm.; average weight about 80 gm. (extremes 60 gm. to 100 gm. or more); specific 

 gravity, 1.047, which is about the same as that of the salivary glands. 



In position, the pancreas lies in the epigastric and left hypochondriac regions. 

 In form, it somewhat resembles a pistol, with the handle placed to the right and 

 the barrel to the left. The pancreas is accordingly divided into a head, lying 

 within the duodenal loop; a bodij, extending to the left; and a tail, or splenic 

 extremity. 



Tlic head [caput pancrcatis] is a discoidal mass somewhat elongated vertically 

 and flattened dorso-vcntrally. It forms the enlarged right extn^mity of the pan- 

 creas and lies within the concavity of the duodenum (figs. 922, 954, 955). Its 

 relations are as follows (figs. 954, 955, 956) : Its posterior surface is placed opposite 

 the second and third lumbar vertebra', and is in contact with the aorta, the vena 

 cava, the renal veins and right renal artery. The common bile-duct is also partly 

 embedded in this surface. Its anterior surface is crossed by the transverse colon, 

 above which is the pyloric extremity of the stomach, and below which are coils of 



