CHANGES IN THE CELLS DURING ACTIVITY. 



397 



The material which accumulates within the cell is not always the same as that 

 which appears in the discharged secretion. Thus in the glands which furnish the 

 digestive ferments especially the gastric glands and the pancreas it has been 



Fig. 453. SECTIONS OF THE ORBITAL GLAND OP THE DOG. A, DURING REST. B, AFTER A PERIOD OK 

 ACTIVITY. (Heidenhain and Lavdovsky.) 



In A, the cells of the alveoli are large, being filled with the material for secretion (in this case, 

 mucigen) which obscures their protoplasm, but some of the cells have not participated in the formation 

 of the secretion, and these remain small and protoplasmic, forming the crescentic group seen in most of 

 the alveoli. 



In B, the accumulated material is discharged from the cells, which appear partially disintegrated in 

 consequence. Both the cells and the alveoli are much smaller, and the protoplasm of all the cells is now 

 apparent. 



shown that the material which appears in the form of granules within the cells is not 

 the pepsin and the trypsin which respectively characterise the secretions of thoss 

 glands, but a precursor which is termed "zymogen," and is easily transformed into 



Fig. 454. PART OF A FERMENT-FORMING GLAND ; A AT REST, B AFTER A SHORT PERIOD OF ACTIVITY, 



C AFTER A PROLONGED PERIOD OF ACTIVITY. (Langley. ) 



In the resting condition the cells of the gland are almost filled with granules (of zymogen). During 

 activity these become discharged from the cells as ferment, disappearing at first from the outer part of 

 the cell, which is thereby left clear. Finally the granules only remain near the lumen of the alveoli, and 

 near the surfaces of the cells which are contiguous to one another. In A and B the nuclei of the cells 

 are obscured by the granules. 



the ferments by the action of certain reagents ; and it is supposed that a similar 

 change may occur during the discharge of the secretion from the cells. Again, in 

 the cells which secrete mucus, the substance which accumulates within the cells is 

 not mucin, but a precursor of mucin, which is termed " mucigen," from the facility 

 with which it becomes on discharge converted into mucin. 



It is difficult to decide whether the material for secretion is formed by the direct 

 conversion of a part of the protoplasm of the secreting cell, or from materials taken 

 up from the plasma of the blood and elaborated by the agency of the protoplasm. 



D D 2 



