SEC. 2. THE ACT OF SECRETION IN THE CASE OF THE 



DIGESTIVE JUICES AND THE NERVOUS MECHANISMS 



WHICH REGULATE IT. 



The various juices whose properties we have just studied, 

 though so different from each other, are all drawn ultimately from 

 one common source, the blood, and they are poured into the ali- 

 mentary canal, not in a continuous flow, but intermittently 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 therefore 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 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 



