70 THE FROG chap. 



their union with the hepatic ducts the common bile duct is 

 formed. 



The bile is manufactured in the liver, and the gall-bladder 

 is merely a reservoir in which it is stored up, to be 

 discharged into the intestine when required for digestion. 



It requires still more accurate observation to show that 

 the pancreas also discharges a fluid, the. panireatic juice, into 

 the intestine. A very delicate branching tube, the pan- 

 creatic duct (/"!), joins the bile-duct, into which it discharges 

 the pancreatic juice, the two fluids entering the intestine 

 together. Both fluids are digestive juices, i.e., liquids which, 

 as we shall see, act in a particular way upon the food. 



By cutting open the enteric canal and 'examining its inner 

 surface under water with a magnifying glass it is seen that the 

 wall of the canal consists of two layers, easily separable 

 from one another. The outer or muscular layer (Fig. 19 A, 

 muse), is tough and strong, the inner layer or mucous 

 membrane {nun), is soft and slimy. Between the two is 

 very loose connective tissue, the submucosa — really a part of 

 the mucous membrane — which, being easily torn, allows of 

 the ready separation of the muscular and mucous layers. 



In the stomach the mucous membrane is raised into 

 longitudinal folds (r), in order to allow of distension ; in an 

 empty stomach they are well marked, and give the cavity a 

 star-like cross-section (Fig. 40) ; in one full of food they are 

 entirely obliterated and the walls of the organ so stretched 

 as to be almost transparent. Anteriorly the ridges thin out 

 and disappear at the cardia or junction between the gullet 

 and stomach ; posteriorly they converge, as the stomach nar- 

 rows, towards the/j'/w7« (Fig. 19 A, /_>•), or junction with the 

 duodenum.' Here the muscular coat is greatly thickened in 

 a ring-like foim, forming the pyloric valve {py.7'), by which 

 the aperture of communication between the stomach and 



