DIGESTION IN THE MOUTH 377 



The total osmotic pressure of the blood is 4,500 mm. Hg, whereas 

 that of the saliva is but 3,000 mm. Hg. 



During the process of secretion there is a greatly increased tissue 

 respiration within the gland, the amount of oxygen used up and the 

 amount of carbon dioxide given out being markedly increased. 



GASEOUS EXCHANGE. 



#2 taken up. G0 2 Output. 



Resting gland .. .. 0-25 c.c per minute. 0-17 c.c. per minute. 



Active gland . . . . 0-86 0-39 



The blood-flow may be increased seven to ten times during secre- 

 tion, and the amount of lymph formed goes up markedly. 



From these considerations, the conclusion is reached that the saliva 

 is not a nitrate or transudate, but a product of the activity of the 

 gland cells, separated by forces which cannot at present be grouped 

 under the physico-chemical processes which have been worked out in 

 the laboratory. 



Certain diffusible substances in the blood sugar, for example 

 never appear in the saliva, whereas certain salts, if taken into the 

 body, are picked out by the salivary glands and secreted in the saliva. 

 This is the case with iodides and mercury. This fact must be borne 

 in mind in prescribing these drugs. 



The secretion of iodides is sometimes made use of to test the 

 absorbing power or motility of the stomach. Potassium iodide is 

 given by the mouth, and the time taken for the appearance of iodides 

 (tested for by starch solution and chloroform) in the saliva noted. 

 The rate of appearance varies with the absorptive power of the stomach 

 for iodides. It is questioned whether this gives any guide for the 

 ordinary absorptive power. To test the motility, the iodide is given 

 in a capsule, which is not digested by the gastric, but by the pancreatic 

 fluid, and the time taken until the iodide appears in the saliva.. The 

 test cannot be regarded as by any means accurate, since its passage 

 out of the stomach is rather a haphazard matter; but should the 

 iodides appear within two to three hours, the motility of the stomach 

 is regarded as normal. 



Heat is produced in the gland, as is shown by the oxygen use and 

 production of carbon dioxide, but, owing to the velocity of the circu- 

 lation, it is not possible to observe the difference in temperature. It 

 has been claimed that the saliva is one or two degrees warmer than the 

 blood; but if the bulb of a very delicate thermometer be introduced 

 into the aorta through the femoral artery, and the bulb of another 

 thermometer is placed in the cannula attached to the salivary duct, 

 and this cannula be filled with water of the same temperature as the 

 blood, then on stimulating the secretion no change in temperature is 

 observed. Electrical changes (see later, p. 566) are also observed in 

 the gland. The current of rest gives place to a current of action, 

 but the direction of the current is different according as the chorda 

 tympani or cervical sympathetic is excited. 



