942 THE EXTERNAL SECRETIONS 



ing of these secretions. The cystic duct is enlarged peripherally into 

 a vesicular receptacle, known as the gall-bladder, but this diverticu- 

 lum is not present in all animals, its place being taken by the biliary 

 ducts themselves which are then distended into tubular pouches. A 

 similar enlargement of these ducts results in those human beings who 

 have had their gall-bladder removed for the relief of malignant and 

 other affections of this organ. Moreover, it should be noted that 

 those animals which are not in possession of this storehouse for bile 

 (horse), show a rather continuous digestive activity and require, there- 

 fore, a more constant supply of bile. 



This is not the case in the carnivora, and hence, the bile is stored in 

 these animals during the periods intervening between the successive 

 periods of digestion. Thus, while the hepatic cells furnish a constant 

 supply of bile, the latter is not conveyed directly into the duodenum, 

 but is diverted through the cystic duct into the gall-bladder. Its 

 storage is made possible by the fact that the orifice of the common duct 

 is guarded by a transverse band of smooth muscle tissue which acts 

 as a sphincter and prevents its constant escape. Naturally, the con- 

 tinuous influx of bile from the hepatic duct gives rise to a gradual disten- 

 tion of the gall-bladder until a stimulation results which relaxes this 

 sphincter and relieves this organ of its contents. Thus, we are really 

 in possession of two separate mechanisms, one for the secretion and 

 one for the storage and expulsion of the bile. 



According to Burton-Opitz,^ the gall-bladder is innervated by 

 fibers derived from the celiac ganglion of the solar plexus which ascend 

 along the small artery, supplying this organ and neighboring region 

 of the liver. Their function becomes evident if it is remembered that 

 the wall of this receptacle is made up in part of smooth muscle tissue 

 which on contraction lessens its lumen and subjects its contents to 

 a moderate pressure. The latter, however, rarely exceeds 5.0 mm. 

 Hg, but evidently, a greater expelling force is not called for, owing 

 to the fact that practically no resistance need be overcome. The 

 pressure at the orifice of the common duct is about zero, unless raised 

 momentarily by peristalsis, so that the only other prerequisite for a 

 free discharge of the bile is the relaxation of the sphincter. It may 

 rightly be concluded that the contraction of the gall-bladder coincides 

 with the relaxation of the latter, although it is not definitely known 

 how this simultaneous action is brought about. It has been estab- 

 lished, however, that it takes place shortly after the entrance of 

 chyme into the duodenum, i.e., generally during the third hour after 

 a meal, but no clear picture can be drawn of the mechanism involved 

 in this process. For the present, therefore, it must be regarded as a 

 reflex evoked in the duodenum, the local nerve paths of which are 

 contained in the plexus celiacus, plexus hepaticus and plexus gastro- 

 duodenalis. Preganglionically, the motor fibers of the gall-bladder 



1 Am. Jour, of Physiol., xlv, 1917, 62. 



