46 THE HUMAN MECHANISM 



We have now to inquire what it is that happens in the 

 gland when it is stimulated by a nervous impulse. 



5. The response of the gland to stimulation by its nerve. 

 The visible result of stimulation of the gland is the discharge 

 of saliva into the mouth. Something must have happened in 

 the gland which has led to the passage of water and other 

 substances from the blood (and lymph) through the gland 

 cells into the duct. But something more has happened, for 

 saliva contains several substances which are not found in the 

 blood. The gland has evidently contributed something to the 

 saliva. How were these contributions to the secretion made? 



FIG. 32. Diagram showing the granules in a resting gland (A) and in a 

 worked alveolus of a gland (E) 



When a gland has been resting for some time microscopic 

 study shows that the cytoplasm of its cells becomes loaded 

 with small granules, at times so numerous as to obscure the 

 nucleus itself. As secretion goes on these granules disappear 

 from the cell, presumably contributing something to the 

 secretion. If the secretion continue for several hours, it is 

 found that the granules have disappeared and that the cell 

 is often distinctly smaller in size than before secretion began. 



The " resting " gland is therefore by no means an idle 

 gland, but gradually stores within its cytoplasm something 

 in the form of granules, which under the influence of 

 nervous impulses or other forms of stimulation more or less 

 rapidly disappears in the secretion. 



6. Activity of the gland involves chemical change within its 

 cells. It might be supposed that the granules manufactured 

 during rest are merely dissolved or washed out of the cells 



