224 Theories of Secretion 



behaves when tested by the close intra-arterial injection technique 

 (Burgen, Terroux and Gonder, 1959). In Fig. 10.16 it is seen that 

 urea crosses the ducts at a point proximal to sodium. Indeed, it 

 enters at the most proximal permeability site yet encountered. The 

 rate of decline of the curve is also slower than encountered with 



10 20 30 40 50 60 70 SO 



sec 



Fig. 10.16. Simultaneous injection of Na 22 and C 14 urea close arterially 

 into the parotid gland of the dog. 



Urea ( ) trails the Na ( — ) by 2-5 seconds at the take off and also declines more slowly 



(Burgen, Terroux and Martin, i960). 



sodium and this is presumably due to the presence of an acinar 

 component. This was to be expected in view of the relatively low 

 duct permeability for urea found in the experiments of Burgen 

 and Seeman. It is perhaps unwise to assume without experimental 

 proof that other non-electrolytes will behave in a way predicted 

 from these relationships, because our experience has been that each 

 new substance studied reveals some detail of the secretory pro- 

 cess that had not been known previously. 



EVIDENCE CONCERNING THE BLOOD SUPPLY 

 OF THE GLAND 



It has been mentioned elsewhere (page 19) that the high clear- 

 ance rate for iodide in the parotid gland of the dog and man, to- 

 gether with the evidence showing that iodide secretion occurs ex- 

 clusively in the duct system of the salivary gland, must be inter- 

 preted to mean that the blood flow to that part of the ducts 

 secreting iodide is very high and probably accounts for a large 

 proportion of the total gland blood flow. In some experiments, 



