Evidence from Changes in Composition 219 



much more mature histologically but the bicarbonate concentration 

 remains very low. These observations suggest that in the immature 

 animal, water and electrolyte secretion might largely originate in 

 the salivary duct system and that the secretion could become 

 hypotonic under these circumstances. Bicarbonate secretion ap- 

 pears to be a process maturing later and is associated with a 

 reciprocal depression of the chloride concentration rather than any 

 change in the cations. Potassium secretion stays remarkably con- 

 stant at all stages of development ; a high concentration of potas- 

 sium in saliva seems to be one of its most universal and stable 

 features. 



The effect of alterations in arterial pCO 2 on saliva bicarbonate and 

 other ions. When the arterial pC0 2 is reduced by hyperventilation, 

 the saliva bicarbonate falls ; however, the total anion concentration 

 in the saliva does not usually change markedly. The reduction in 

 bicarbonate is accompanied by an equivalent increase in the chloride 

 concentration. Conversely, when the pC0 2 is increased, the bicar- 

 bonate in the saliva rises and the chloride falls, although frequently, 

 if the saliva carbonate has reached a very high level, some increase 

 in sodium concentration also occurs. This evidence also suggests 

 that the concentrations of bicarbonate and chloride in saliva are 

 intimately related and that probably bicarbonate enters the saliva 

 in exchange for chloride reabsorbed and that this exchange pro- 

 cess is controlled by the arterial pC0 2 . In some animals, however, 

 changes in bicarbonate are accompanied mainly by changes in 

 sodium concentration. It is not known what favours one process 

 rather than the other. 



Effects of adrenaline. It was found by Langstroth, McRae and 

 Stavraky (1938) that following a relatively large does of adrenaline, 

 the inorganic composition of cat submaxillary saliva evoked by 

 subsequent chorda stimulation was changed. The major altera- 

 tion is a considerable increase in total osmolyte concentration, 

 accounted for almost wholly by an increase in sodium and chloride. 

 Neither potassium nor bicarbonate are greatly affected. Martin 

 (i960) has shown that at least part of this effect is exerted on the 

 salivary duct system and involves an increase in permeability for a 

 number of substances including amino-acids, sucrose, creatinine 

 and urea. The lack of effect on bicarbonate suggests that in this 

 case sodium and chloride are behaving as an ion pair and that the 

 effect of adrenaline is either to inhibit their reabsorption from the 



