456 DIGESTION 



the permeability of the lumen border of the cell. This change in permea- 

 bility may be dependent upon alterations in surface tension brought 

 about by the migration of electrolytes to the border. That such a migra- 

 tion of electrolytes does actually occur has been demonstrated by A. B. 

 Macallum who developed a microchemical test for potassium, by the use 

 of which he was able to show that this electrolyte accumulates at the lumen 

 border of the cell during secretory activity, that is, at the border of the 

 cell through which the secretion takes place. Potassium may be taken 

 as a prototype of electrolytes in general. In the epithelium of the small 

 intestine, where the current goes in the opposite direction to that in 

 gland cells, the accumulation of potassium occurs at the portion of the 

 cell next the basement membrane. 



Other observers believe that, when the gland becomes more active, the 

 molecules present in the cell become broken down into smaller molecules 

 and so raise the osmotic pressure of the cell content, with the result that 

 water is attracted from the blood and is then transferred to the lumen. 

 When the gland is excited so that the zymogen granules, as well as 

 water and salts, are secreted, the primary change appears to involve the 

 granules only. Those near the lumen swell up by absorbing water, and 

 become converted into spheres in which salts are dissolved in smaller 

 proportions than exist in the lymph bathing the cells. These swollen 

 structures are then ruptured at the periphery of the cell and discharged 

 into the lumen. This discharge of a fluid containing fewer saline con- 

 stituents than the cell or surrounding blood plasma brings about in- 

 creased concentration in the remaining parts of the cell, a process which 

 possibly is assisted by a breaking up of molecules in the protoplasm itself, 

 and which causes an increase in osmotic pressure with a consequent 

 flow of water from the lymph to the cells and therefore from the blood 

 to the lymph. 



OTHER CHANGES DURING ACTIVITY 



Whatever may be the nature of the physiological changes that 

 are responsible for the secretory activity of the cell, the fact stands out 

 prominently that a considerable expenditure of energy is entailed. This 

 is indicated by the fact that considerably larger quantities of oxygen 

 are taken up by the gland when it is in an active state than when at 

 rest. Thus, the oxygen consumption of the resting submaxillary gland 

 of the cat may be increased five times during active secretion. On 

 account of this increased oxygen consumption it is not surprising that 

 it should be found that the secretory activity of the cell is greatly im- 

 paired by a deficiency in oxygen. 



