268 TEE HUMAN BODY. 



in which its activity is very little, but during which it is 

 abundantly supplied with food materials. The pancreas 

 taken from an animal at the end of the first period and 

 prepared for microscopic examination will be found dif- 

 ferent from that taken from a dog killed at the end of the 

 second digestion period, and also from the resting gland. 

 Towards the end of the period of active work, the gland-cells 

 are diminished in size and the proportions of the granular 

 and non-granular zones are quite altered. The latter now 

 occupies most of the cell, while the granular non-staining 

 inner zone is greatly diminished. During the secretion 

 there is, therefore, a growth of the non-granular and a de- 

 struction of the granular zone; and the latter process rather 

 exceeding the former, the whole secreting cell is diminished 

 in size. During the second digestive period, when secre- 

 tion is languid, exactly a reverse process takes place. The 

 cells increase in size so as to become larger than those of 

 the resting gland; and this growth is almost entirely due 

 to the granular zone which now occupies most of the cell. 



These facts suggest that during secretion the granular 

 part of the cells is used up: but that, simultaneously, the 

 deeper non-granular zone, being formed from materials 

 yielded by the blood, gradually gives rise to the granular. 

 During active secretion the breaking down of the lat- 

 ter to yield the specific elements occurs faster than its re- 

 generation; in a later period, however, when the secretion 

 is ceasing, the whole cell grows and, especially, the granular 

 zone is formed faster than it is disintegrated; hence the 

 great increase of that part of the cell. If this be so, then 

 we ought to find some relationship between the diges- 

 tive activity of an infusion or extract of the gland and the 

 size of the granular zones of the cells; and it has been 

 shown that such exists; the quantity of trypsin which can 

 be obtained from a pancreas being proportionate to the 

 size of that portion of its cells. 



The trypsin, however, does not exist in the cells ready 

 formed, but only a body which yields it under certain cir- 

 cumstances, and called trypsogen. 



If a perfectly fresh pancreas be divided into halves and 



