CHANGES IN THE GLAND CELLS. 



147 



aeti ve secretion be continued for some time, the cells return to their 

 former protoplasmic state, and the cells which have been worn 

 out are replaced by others from the demi-lune or marginal cells. 



In the glands which do not produce any mucus the brilliant 

 look of the cells after rest is wanting, but a corresponding change 

 occurs. The secreting protoplasm becomes extremely granular 

 during the resting period, and again clear after the discharge of 

 the secretion. (Fig. 67.) 



Thus it would appear that during the so-called period of rest, 

 when little or no fluid is poured into the duct, the gland cells are 

 busy at their manufacturing process, diligently adding to their 

 stock in hand in order to be ready for a certain demand which 

 they could not meet by merely concurrent work. 



Cells of the Alveoli of a Serous or Watery Salivary Gland. (Langley.) (A) After rest. 

 (B) After a short period of activity, (c) After prolonged period of activity. 



To sum up, then, we may conclude : 



I. That the manufacture of the specific materials of the 

 secretion is accomplished as the result of the intrinsic 

 power of the protoplasm of the gland cells. 

 II. That a vital process is called forth in the gland cells by 

 the action of nerve impulses, because (1) The force 

 with which the secretion is expelled cannot be ac- 

 counted for by the blood pressure. (2) The quantity 

 and quality of the secretion is modified by the inten- 

 sity of the nerve stimulation. (3) The temperature 

 of the blood is raised. (4) Structural changes in the 

 cells can be observed. 

 III. The normal stimulus to secretion passes from the centre 



