170 BIOLOGICAL LECTURES. 



is this brought about ? It is clearly due to the fact that the 

 cell membrane is more pervious or less resistant in one part 

 than another. This is a matter that must necessarily be prop- 

 erly arranged, or the cell would discharge first in one direction, 

 then in another. In nearly all glands the cells are so arranged 

 that the weakest spot is toward the gland lumen and away from 

 the basement membrane. In many cases, however, nearly the 

 whole surface except that lying against the basement membrane 

 permits the escape of the secreted material. This is doubtless 

 the explanation of such pictures as one often sees in Golgi 

 preparations of the pancreas and salivary glands, in which the 

 impregnation of the so-called "secretory capillaries" extends 

 between the cells nearly to the basement membrane. It is 

 probably owing to this differentiation of the cell and the uneven 

 resistance of the limiting membrane that cells will secrete even 

 osmosable substances in one direction only. Consider, for 

 example, the soluble bile salts of the liver cells which leave the 

 cells always by the biliary capillaries. Probably where the bile 

 capillary enters the cell is the point of least resistance, whither 

 the bile salts must journey as fast as they are formed. The 

 cell leaks, as it were, at this spot. 



Nothing is more obvious than that the processes of secretion 

 are under some sort of a control. Many cells and glands do 

 not secrete constantly, but only when needed. They secrete 

 intermittently. How is this intermittence brought about ? 

 How is secretion controlled ? We shall find it necessary to 

 distinguish in the case of multicellular glands between the dis- 

 charge of secretion from the gland duct and its discharge from 

 the cell. It by no means follows that when the cell is secret- 

 ing, the secretion is escaping from the duct ; nor, on the other 

 hand, that when the gland is secreting, the cells of the gland 

 are secreting also. The two processes often coexist, but this 

 is by no means invariably, one might say even generally, the 

 case. A great deal of confusion unfortunately existing in our 

 conceptions of secretion arises from a failure to take account 

 of this fact. In a great many cases, perhaps the majority of 

 cases, the secretion from the cells is continuous, while the dis- 

 charge from the duct is intermittent. In other well-recognized 



