422 THE DIGESTIVE SYSTEM 



The fibroserous coat consists of loose areolar tissue, which contains 

 the larger blood-vessels with which the organ is abundantly supplied. 

 The free surface of the gall-bladder also receives a peritoneal investment. 



Occasional mucous glands occur in the mucosa of the gall-bladder. 

 These are mostly of small size and widely separated, but toward the neck 

 of the organ they increase in both number and size. They form short, 

 branched, convoluted tubules. 



The blood-vessels form a plexus just outside the muscular coat, from 

 which branches are distributed to the peritoneal coat and to a plexus in 

 the depth of the mucosa from which capillaries are supplied to the muscu- 

 lar layers and to a subepithelial plexus. The nerves are distributed to 

 the blood-vessels and to the muscular wall. Minute ganglia occur in the 

 muscular coat. 



The gall-bladder is lacking in many species of vertebrates includ- 

 ing the lamprey, pigeon, rat. horse, certain ruminants and the porpoise. 

 In these instances a well-differentiated cystic anlage suffers regressive 

 changes in later embryonic life and ultimately disappears. With the 

 disappearance of the gall-bladder, the biliary system generally under- 

 goes compensatory enlargement. Dilatation of the bile ducts is fre- 

 quently associated with congenital absence of the gall-bladder in the 

 human subject, and commonly follows cholecystectomy (see Scammon, 

 Anat. Rec., 10, 8, 1916). 



