80 PHYSIOLOGY CHAP. 



adopted by Langley, is more probable, and harmonises with the 

 fact that, generally speaking, living cells in carrying out their 

 functions consume the chemical matters which they have 

 absorbed and elaborated, and only utilise their own protoplasm 

 when all other materials are exhausted. 



IV. The important question of the specific property by which 

 the salivary glands select from the substances offered them by the 

 blood the constituents of an effusion that is quite unlike the blood 

 itself, has hitherto been treated only by a few authors, and that 

 incidentally. Novi (1888) estimated the amount of chlorine 

 contained in a sample of blood from the carotid and in one of 

 saliva, before and after injecting a 10 per cent solution of sodium 

 chloride into the jugular. He found that when the concentration 

 of the blood was thus increased, the rate of secretion increased 

 also. Novi further observed that the chloride content of the 

 saliva increased much more rapidly than that of the blood serum. 

 When, e.g., the chloride in the sodium increased from 100 to 155, 

 that in the saliva increased from 100 to 220. Langley and 

 Fletcher confirmed the observations of Novi, showing that dilute 

 solutions of sodium chloride, while they still augment the rate of 

 secretion, lower the concentration of the saliva. 



Asher and Cutter (1900) on injecting sugar and urea showed 

 that sugar excites secretion only by causing hydraemic plethora, 

 and does not appear in the saliva ; urea, on the contrary, excites it, 

 and partly reappears in the saliva. 



According to Aducco these different effects show that the 

 production of the secretion depends not only upon the physical 

 and chemical constitution of the circulating substances, but also 

 upon their effect on metabolism. Thus urea, which is a katabolic 

 product, and is not present in the normal secretion of the gland, is 

 capable of activating it and of stirring up the .secretory cells to 

 more work ; while sugar does not pass through the gland (provided 

 the physiological limits are not exceeded), and only acts indirectly 

 upon the secretion, i.e. by augmenting the mass and the dilution 

 of the blood. 



This explanation of the phenomenon appears to us inadequate. 

 If under the said experimental conditions it is a fact that the 

 increased sugar content of the blood increases the flow of saliva, 

 under other conditions, e.g. in experimental hyperglycaemia and 

 in diabetes in general, the secretion of saliva is very scanty, much 

 below the normal. 



Many salts when introduced into the vascular circulation 

 appear rapidly in the saliva (potassium iodide, lithium citrate, etc.) ; 

 others, on the contrary (bile salts), which are eliminated by all 

 other glands when present in the blood do not pass through 

 the salivary glands. 



In regard to the selective capacity of the salivary glands, 



