240 THE MECHANISM OF LIFE 



blood to the cells as the circumstances require. How do the 

 secretory fibres act on the cells ? When the latter function, it 

 can actually be seen that granules of some material taken from 

 the blood have been formed and deposited in their substance. 

 The stimuli of the secretory fibres seem to cause these granules 

 to disintegrate, absorb water, and swell, finally discharging their 

 contents into the cavity of the alveolus. This is the thing, then, 

 that has to be " explained." 



What is the substance of the granules ? How is it taken from 

 the blood ? How is it liberated into the alveolus when the cell 

 is stimulated ? How is it that the nature of the substance varies 

 according to the kind of food ? How can the sight of food 

 produce the same effect as the taste or smell of food ? How can 

 the memory of food do the same thing as the taste of food — that 

 is, how does an immaterial, non-energetic stimulus produce the 

 same response as an energetic, material one ? 



It will be seen that what we have attained so far is a rather 

 imperfect description of the mechanism of control of the secretion 

 of saliva. So far as the act of secretion itself is concerned, 

 physiology has succeeded in substituting a biochemical descrip- 

 tion for a mechanical one, but even the biochemical description 

 involves at the pre3ent time hypotheses that have still to be 

 verified. 



