420 DIGESTION 



out any change in the arrangement of the granules. In such cases it has 

 been observed, as in the pancreas, that fine channels develop in the 

 protoplasm of the cell (see page 429). 



From this histological evidence it would appear that the gland cell 

 during rest is endowed with the property of building up out of the pro- 

 toplasm, as granules or globules, the material which is to serve as one of 

 the main organic constituents of the secretion. It is commonly believed 

 that this is the precursor of the active ferment of the secretion ; hence its 

 name, zymogen. It has been shown that the process of separation of the 

 zymogen granules starts around the nucleus with the production of a 

 basophile substance, which in hardened specimens sometimes takes the 

 form of filaments. From this basophilic ergastoplasm, as it is called, the 

 granules are gradually formed, and then for some time continue to 

 undergo slight further changes, as is evidenced by the fact that the 

 staining reaction of those near the base of the cells differs from that of 

 those at the free margin. "When the gland cell is excited to secrete, 

 the granules before being extruded, as noted above, often undergo a 

 definite change, becoming swollen and more globular in shape. 



MECHANISM OF SECRETION 



These microscopic studies merely tell us that active changes, associated 

 with the production and liberation of certain of the constituents of its 

 secretion, are occurring in the gland cell, but they throw no light on the 

 mechanism whereby the gland cells secrete water and inorganic salts. 

 This may be dependent, to a certain extent at least, on differences in 

 osmotic pressure (see page 11). A possible explanation of the flow of 

 water is as follows: If a watery solution of some osmotically active sub- 

 stance is put in a tube, which is closed at one end by a membrane 

 impermeable to this substance and at the other by one permeable to it. 

 and the tube immersed in water, a continuous current will be 

 found to issue from the permeable end so long as there remains any 

 osmotically active substance in the tube. If we assume, then, that the 

 membranes at the two ends of the secreting cell are of such a nature that 

 the one next the basement membrane is impermeable to some osmotically 

 active substance manufactured by the cell, and the other toward the 

 lumen is permeable, it will be clear that, so long as this substance 

 exists in the cell, it will attract water from the blood, and the water 

 together with the osmotically active substance will be discharged into 

 the lumen. 



It is possible that when anything excites the cell to secretory activity, 

 such as a nerve impulse or hormone, it does so by causing a change in 



