Glandular Secretion of Electrolytes 69 



becomes profuse (Araki and Ando, 1953; Bulmer, 1957). In 

 parotid saliva the ratio decreases from about 1 • 6 at low rates 

 of secretion to about 0-6 when the flow of saliva is brisk 

 (Albrectsen and Thaysen, 1955). Since no specific secretory 

 mechanism for urea exists in either gland, it is reasonable to 

 conclude that urea, which is diffusing into the gland with 

 some precursor solution, is raised to a concentration greater 

 than that of the plasma by reabsorption of water from the 

 precursor in a region of the gland which is less permeable to 

 urea than the site of precursor formation. The rate of change 

 in S/P urea with secretory rate suggests that water reabsorp- 

 tion represents a relatively constant quantity at all rates of 

 precursor formation, and it is not unreasonable to assume 

 that the reabsorption of water occurs as a mere passive 

 sequence of active sodium reabsorption. 



Quantitative information about precursor formation and 

 water reabsorption can, however, hardly be gained from these 

 results or from similar "clearance" studies with other solutes. 

 Morphological and physiological evidence strongly argues 

 against the possibility that the secretion precursor represents 

 an ultrafiltrate of the plasma like the urine precursor of the 

 glomerular nephron. A "glandular inulin" probably does not 

 exist, and it is quite possible that exact knowledge about 

 the composition of the precursor secretions and about the 

 manner in which they are modified as they flow down the 

 glandular ducts can only be obtained by micropuncture 

 techniqiies. 



However, Lundberg (1955, 1957«,&,c), working on the 

 electrophysiology of the submaxillary and sublingual glands 

 of the cat, has obtained results which provide indirect support 

 in favour of the hypothesis that sodium is reabsorbed from a 

 precursor secretion in some of the duct-possessing glands. 



In the submaxillary gland, which produces a secretion in 

 which sodium concentration varies with secretory rate in 

 about the same manner as in parotid saliva and sweat, 

 Lundberg (1955) demonstrated that the lumen of the (striated?) 

 ducts becomes negative as compared to the hilus, when the 



