CHAPTER II 



EXTERNAL DIGESTIVE SECRETIONS 



CONTENTS. 1. Structure of salivary glands : cranial and sympathetic inner - 

 vation. 2. Nervous mechanism of secretion in salivary glands. 3. Cytological 

 changes in secretory epithelium during rest and secretion. 4. Selective activity 

 of salivary glands. 5. Chemical analysis of salivary glands and the various 

 kinds of saliva. 6. Structure of pancreas. 7. Innervation and mechanism 

 of its secretion. 8. Pancreatic juice. 9. Internal function of the pancreas. 

 10. Factors concerned in internal pancreatic secretion. 11. Structure of gastric 

 mucosa and glands. 12. Innervation. 13. Gastric juice and the cells which secrete 

 it. 14. Zymogens which give rise to the gastric enzymes. 15. Intestinal glands. 

 16. Succus entericus. 17. Mechanism of intestinal secretion. 18. Structure of the 

 liver. 19. Secretion of bile in digestion and fasting. 20. Influence on secretion 

 of changes in hepatic circulation. 21. Chemical constituents of bile. 22. Origin 

 and metabolic activity of hepatic cells. Bibliography. 



FROM the group of glands which have no excretory duct, and 

 can only serve for internal secretion, we must distinguish the 

 group that are provided with excretory ducts, and are therefore 

 capable of external secretion, their products being poured out into 

 the gastro - intestinal canal. The principal function of these 

 secretions is that of chemical and physical transformation of 

 the ingested food, so as to render it fit for absorption and assimila- 

 tion, and thus to repair the losses perpetually sustained by the 

 tissues during the exercise of their functions. 



These organs of external digestive secretion may histologically 

 be divided into three groups : (a) acinous glands, forming distinct 

 organs (salivary and pancreatic) ; (&) tubular glands, scattered in 

 the depth of the mucosa of the digestive tube (buccal, gastric, 

 and intestinal) ; (c) glands with branching tubes, grouped into a 

 large organ which forms the liver. 



The external secretion, which (with the exception of the 

 biliary secretion) is the chief function of these organs, does not 

 exclude them from serving for internal secretion also ; but this 

 subject will be discussed in a subsequent chapter, the better to 

 appreciate its nature and physiological significance. 



I. The excretory ducts of three principal pairs of glands which 

 manufacture and secrete Saliva open into the buccal cavity. 

 From their position these were termed parotid, sub-maxillary, and 



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