Glandular Secretion of Electrolytes 71 



glands. In some glands the chief anion transported is chloride 

 (sweat, tears, sublingual saliva); in others bicarbonate ions 

 are added in varying proportion, possibly due to the presence 

 of carbonic anhydrase in the cells (pancreatic juice, parotid 

 saliva, submaxillary saliva). It is reasonable to assume that 

 water moves in a merely passive sequence of ionic transport 

 from the blood side into the glandular lumen, and that the 

 presecretions of all glands are isotonic or nearly isotonic. 



In certain glands (sweat, parotid and submaxillary) sodium 

 is reabsorbed from the precursor secretion as it flows down the 

 glandular duct system, and it is likely that anions move from 

 duct lumen to the blood side in a passive sequence of the 

 active sodium reabsorption. The chief anion reabsorbed in 

 this manner appears to be chloride, independently of whether 

 the primary secretion contains primarily chloride or primarily 

 bicarbonate ions. It can be seen from a glance at Fig. 1 that 

 the parotid and the pancreatic glands apparently form pre- 

 secretions of qualitatively similar composition, and that the 

 main difference in the anionic pattern of the final secretory 

 products is that chloride ions have been removed from the 

 saliva precursor. As a consequence of active sodium re- 

 absorption a certain quantity of water is, moreover, diffusing 

 back into the blood stream, although it is obvious that 

 water reabsorption does not occur isotonically as in the 

 proximal renal tubule. 



It is only possible to speculate on the morphological sites of 

 the different ionic transports in the duct-possessing glands. 

 According to Fig. 5 it is, however, not unreasonable to 

 suggest that sodium reabsorption is located in the striated 

 intralobular ducts. Striated epithelium is present in the 

 parotid and submaxillary glands, which apparently reabsorb 

 sodium, but it is absent in the sublingual, pancreatic and 

 lachrymal glands, which show no evidence of sodium re- 

 absorption. The precursor secretions are probably formed 

 by the acini as well as by the cuboidal epithelium of the 

 intercalary ducts, the former producing a viscous secretion 

 with a high concentration of organic material, the latter a 



