SEC. 2. THE STRUCTURE OF THE SALIVARY GLANDS, 

 THE GASTRIC MUCOUS MEMBRANE, THE PANCREAS, 

 AND THE (ESOPHAGUS. 



208. Before we study the nature of the processes by which 

 the stomach and the salivary glands are able to secrete the gastric 

 juice and saliva, whose remarkable properties we have just described, 

 it will be desirable to say a few words on the structure of both the 

 above organs. 



Throughout the greater part of its length, from the cardiac 

 end of the oesophagus to near the anus, the alimentary canal is 

 constructed on a certain general plan. This part of the alimentary 

 canal is formed out of the mid-gut of the embryo, and the epithelium 

 which lines it is of hypoblastic origin. The mouth and the anus 

 have a different origin; they are formed by involutions of the 

 external skin, the epithelium of which is of epiblastic origin ; and 

 the plan of structure of the mouth and terminal portion of the~ 

 rectum is in .some respects different from that of the rest of the 

 alimentary canal. In the adult the transition from epiblastic to 

 hypoblastic features occurs in the rectum at the anus, but at the 

 other end an epithelium with distinctly epiblastic characters 

 extends for some distance from the mouth, as far indeed as the 

 junction of the oesophagus with the stomach. 



The plan of structure of the hypoblastic portion of the canal is 

 somewhat as follows. 



A single layer of cylindrical, columnar, cubical or spheroidal 

 "protoplasmic" cells, that is to say cells which are not transformed 

 into flattened scales, forms the immediate lining of the cavity. 

 The cells rest on a connective tissue basis, which is fine, delicate 

 and often of a peculiar nature immediately under the epithelium, 

 but becomes more open, loose and coarse at some little distance 

 from the cells. This connective tissue basis is richly provided, 

 with blood vessels and lymphatics, and also contains a certain 

 number of nerves. The blood vessels reach up to, and fine 

 capillary networks are especially abundant immediately beneath, 

 the bases of the cells, but none pass between the cells themselves; 



