130 THE WORK OF THE DIGESTIVE GLANDS. 



there are many territories where the reciprocal relations of the in- 

 dividual component parts of a system (for example, the digestive canal) 

 and the bearing of the whole towards objects of external nature (in 

 this case to the food) will receive a complete explanation. At the very 

 portals of organ-physiology we find such questions as, What is the 

 peripheral end -apparatus of a centripetal nerve ? How does it perceive 

 this or that form of excitation ? What are the nervous phenomena by 

 which reactions and molecular changes in the secretory cell lead to the 

 formation of this or that ferment, or to the preparation of this or that 

 reagent? We have hitherto taken these properties and elementary 

 functions as granted, and investigated the rules and laws of their 

 working in the apparatus as a whole. We have learned how this or 

 that apparatus may be set into activity ; that is to say, within certain 

 limits we have been able to understand it. 



