70 THE FROG CHAP. 



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 pancreatic 

 juice, into the intestine. A very delicate branching 

 tube, the pancreatic duct (P 1 ), 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 parti- 

 cular 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], covered by the peri- 

 toneum (p. 27), is tough and strong, the inner layer or 

 mucous membrane (m.m), 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 these 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 narrows, 

 towards the pylorus (Fig. 19, A, py}, or junction with 

 the duodenum. Here the muscular coat is greatly 

 thickened in a ring-like form, forming the pyloric valve 

 (py. v), by which the aperture of communication between 

 the stomach and intestine is greatly narrowed, so that 



