212 DIGESTION. 



reaction with the perchloride of iron. By suitable manipulations, the presence of sul- 

 phur was also established. 



Longet states, farthermore, that he has examined the saliva from a great number of 

 persons, under all conditions, and has never failed to demonstrate the presence of the 

 sulpho-cyanide. Its proportion he found very variable, and in some cases it was so 

 slight that the reaction with the perchloride of iron did not immediately manifest itself; 

 but, by slowly evaporating the liquid to one-half or one-third of its original volume, 

 the reaction was observed in all cases. 



It is probable that the sulpho-cyanide of potassium is a constant ingredient of each 

 of the three varieties of saliva. It has been found in the parotid, in cases of salivary 

 fistula, and was noted by Dalton in the saliva taken from the duct of Steno, although, in 

 this case, the saliva contained an organic principle which interfered with the test, but 

 which could be precipitated by alcohol and separated by filtration. Longet found the 

 sulpho-cyanide in the saliva from the submaxillary and sublingual glands, taken from the 

 floor of the mouth behind the inferior incisor and canine teeth. 



Very little need be said concerning the remaining inorganic constituents of saliva, 

 except that they are of such a nature as almost invariably to render the fluid distinctly 

 alkaline. They exist in small proportion and do not appear to be connected in any way 

 with the functions of the saliva as a digestive fluid. 



Functions of the Saliva. 



Physiologists are not entirely agreed concerning some of the most important questions 

 relating to the function of the mixed saliva in digestion. Bernard, from observations on 

 the lower animals, particularly on dogs, concludes that the operation of the saliva is simply 

 mechanical ; while others, in view of its property of rapidly transforming starch into 

 sugar, attribute to it an important chemical function. The experiments on which the 

 view of Bernard is based are conclusive, so far as they go. He has shown that none of 

 the distinct varieties of saliva from the dog affect starch ; that a mixture of the fluids 

 from the three salivary glands is likewise inoperative ; and that the mixed saliva from 

 the mouth of the dog, containing the secretion of the mucous glands of the mouth, con- 

 verts starch into sugar with difficulty. At the same time, however, he mentions the 

 well-known fact that the human mixed saliva changes starch into sugar with great 

 rapidity, and that the same effect is produced by the unmixed parotid or submaxillary 

 secretion. In the dog, amylaceous principles taken by the mouth are always found un- 

 altered in the stomach and are only transformed into sugar in the small intestines ; but 

 observations have shown that this is not the case in the human subject. These facts are 

 a sufficient argument against the direct application of experiments made on an exclusive- 

 ly carnivorous animal, like the dog, to the digestive process in man. While there is no 

 reason to suppose that there is any material difference in the mammalia, as regards the 

 general operation of some of the functions, such as circulation or respiration, it is evident 

 that differences exist in the properties of the digestive fluids, as well as in the teeth and 

 jaws, corresponding with the great differences in the character and conditions of the 

 alimentary principles. In the study of digestion, therefore, the results of experiments 

 on the inferior animals cannot always be taken without reserve, and they should be con- 

 firmed by observations on the human subject ; but, fortunately, the properties of nearly 

 all of the digestive fluids which have been studied minutely by vivisections have been 

 investigated more or less fully in man. 



In 1831, Leuchs discovered that hydrated starch, mixed with fresh saliva and warmed, 

 became liquid in the space of several hours and was converted into sugar. This fact has 

 since been repeatedly confirmed ; and it is now a matter of common observation that 

 hydrated starch or unleavened bread, taken into the mouth, almost instantly loses the 

 property of striking a blue color with iodine and responds to the ordinary tests for sugar. 

 Of the rapidity of this action any one can easily convince himself by the simple experi- 



