184 DIGESTION. [CHAP. xxin. 



which seem to come from the salivary glands or ducts. Its viscidity 

 is increased by mixture with the mucus of the mouth. Accord- 

 ing to Dr. Wright, its specific gravity is, on an average, about 

 1007*9. It is usually alkaline, especially during a meal, but often 

 neutral, and sometimes slightly acid. Less than two parts in a 

 hundred are organic or saline matters, the rest is water. The 

 organic matters are (besides the nucleated cells) ptyalin or salivin, 

 fat (often visible as oil-globules in the microscope ), and extractive 

 matter, with a trace of albumen ; the inorganic constituents are 

 alkaline lactates, chlorides of sodium and potassium, phosphate of 

 lime, some free soda, with sulpho-cyanide of potassium, and perhaps 

 others. The last-named product gives a red tinge with a persalt of 

 iron, and seems- peculiar to the saliva. With regard to the nature 

 and properties of what has been termed ptyalin, chemists appear to 

 be by no means agreed, or even whether it be at all peculiar to this 

 secretion. Dr. Wright, who has paid a great deal of attention to 

 the saliva, describes it as a yellowish-white, adhesive, and nearly 

 solid matter, having alone the characteristic odour of saliva, soluble 

 in ether, alcohol, and essential oils, but more sparingly so in 

 water ; as unaffected by most of the agents which coagulate albu- 

 men, but as abundantly precipitated by subacetate of lead and 

 nitrate of silver. Dr. Franz Simon, on the contrary, describes 

 ptyalin as insoluble in alcohol and ether ; and he adds, " Our know- 

 ledge of this substance is by no means accurate ; and there is no 

 doubt that all the animal fluids yield an extract to water, which 

 strongly resembles, if it be not altogether identical with, ptyalin. "* 

 Dr. Miller has published a recent analysis of the saliva, t 



An obvious use of the saliva is to aid in reducing the food to a 

 pultaceous form, in which it is more easily swallowed. During the 

 movements of mastication, it is intimately mingled with the whole 

 mass, and may thus very probably mechanically enable the gastric 

 juice to penetrate more quickly to every part on its arrival in the 

 stomach. But general experience attributes the ill-effects of rapid 

 eating or bolting of the food to the saliva being swallowed in in- 

 sufficient quantity, as well as to imperfect mastication ; and it is a 

 question of some interest, to ascertain whether this fluid has any di- 

 gestive powers, at all allied to those of the gastric juice. The ex- 

 periments of Leuchs have proved that it has the power of convert- 

 ing starch into sugar, a change similar to that which occurs in the 



* Animal Chemistry, translated by Geo. E. Day (London, 1845), p. 24. 

 t Cyclop. Anat., art. Organic Analysis, p. 812. 



