190 THE DIGESTIVE SYSTEM 



the intestine. The Hver also has important endocrine functions. 

 It forms urea from ammonium carbonate and later on the urea is 

 removed from the l)lood by the kidney and excreted. The most 

 important endocrine function of liver cells invohes taking such 

 a simple carbohydrate compound as orlucose from the blood and 

 changing it to temporarily insoluble glycogen. Then as tissues, 

 especially the muscles, need this fuel the li^'er recon\'erts glycogen 

 back into simple sugar (glucose) in which form it is distributed by 

 the blood-vascular system. The presence of insulin derived from 

 islands of Langerhans in the pancreas is essential to the carrying 

 out of this function. Tests show that li\'er cells, depending on the 

 diet, in some cases show a preponderance of protein products, at 

 other times of glycogen, and at still other times of fats. An extract, 

 called heparin, obtained from liver tissue pre\ents the clotting of 

 blood. Another substance obtained from liver tissue stimulates 

 the production of erythrocytes. Li\'er tissue possesses great power 

 of regeneration, but the remo\'al of the entire organ results in the 

 death of the organism in a short time. 



THE GALL-BLADDER. 



The largest bile ducts from the lobes of the mammalian liver 

 usually connect with a pouch, the gall-bladder, which lies along the 

 posterior under surface of the liver. The cystic duct continues 

 outward to join the common bile duct. The wall of the gall- 

 bladder consists of three coats, a mucosa, muscularis, and an 

 adventitia. The mucosa is much folded and has a co^'ering layer 

 of tall columnar epithelial cells with basal nuclei. The tunica 

 propria carries blood vessels, and toward the neck region of the 

 pouch there occur small tubulo-alveolar glands. The muscidaris 

 possesses interwoven bimdles of smooth muscles arranged as an 

 inner longitudinal layer and an outer thicker circular coat. The 

 outer part of the adventitia is not so dense as the inner and sup- 

 ports blood and lymj^h vessels and nerves. 



In the fish, if present, it has a lining of simple or stratified columnar 

 epithelium surrounded by fibrous connective tissue. In the frog, 

 it is lined with columnar ei)ithelium surrounded by connective tissue 

 which supports blood vessels. In the water snake (Natrix), it is 

 lined with stratified columnar epithelium, and outside this is con- 

 nective tissue with smooth muscle cells in it (lis})ose(l in strands 

 around the wall. 



