SEC. 2. THE ACT OF SECRETION OF SALIVA AND 

 GASTRIC JUICE AND THE NERVOUS MECHANISMS 

 WHICH REGULATE IT. 



186. The saliva and gastric juice whose properties we have 

 studied, though so different from each other, are both drawn ulti- 

 mately from one common source, the blood, and they are poured 

 into the alimentary canal, not in a continuous now, but intermit- 

 tently as occasion may demand. The epithelium cells which 

 supply them have their periods of rest and of activity, and the 

 amount and quality of the fluids which these cells secrete are 

 determined by the needs of the economy as the food passes along 

 the canal. We have now to consider how the epithelium cell 

 manufactures its special secretion out of the materials supplied to 

 it by the blood, and how the cell is called into activity by the 

 presence of food, it may be as in the case of saliva at some dis- 

 tance from itself, or by circumstances which do not bear directly 

 on itself. In dealing with these matters in connection with the 

 digestive juices, we shall have to enter at some length into the 

 physiology of secretion in general. 



The question which presents itself first is : By what mechan- 

 ism is the activity of the secreting cells brought into play ? 



While fasting, a small quantity only of saliva is poured into 

 the mouth ; the buccal cavity is just moist and nothing more. 

 When food is taken, or when any sapid or stimulating substance, 

 or indeed a body of any kind, is introduced into the mouth, a flow 

 is induced which may be very copious. Indeed the quantity 

 secreted in ordinary life during 24 hours has been roughly cal- 

 culated at as much as from 1 to 2 litres. An abundant secretion 

 in the absence of food in the mouth may be called forth by an 

 emotion, as when the mouth waters at the sight of food, or by a 

 smell, or by events occurring in the stomach, as in some cases of 

 nausea. Evidently in these instances some nervous mechanism 

 is at work. In studying the action of this nervous mechanism, it 

 will be of advantage to confine our attention at first to the sub- 

 maxillary gland. 



