SEC. 3. THE ACT OF SECRETION OF SALIVA AND 

 GASTRIC JUICE AND THE NERVOUS MECHANISMS 

 WHICH REGULATE IT. 



224. The saliva and gastric juice whose properties we have 

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

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

 into the alimentary canal, not in a continuous flow, 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 distance 

 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 mechanism 

 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 calculated 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 submaxillary gland. 



