148 FUNCTIONS OF NUTRITION. 



of saliva absorbed. The following table gives the results of some of 

 Lassaigne's experiments upon a horse : 



Kind of Food employed. Quantity of Saliva absorbed. 



For 100 parts of hay 400 parts. 



" barley meal 186 " 



oats 113 " 



" green stalks and leaves . 49 " 



It is evident from the above that the quantity of saliva used in 

 mastication has not so much to do with the chemical character of the 

 food as with its physical condition. When the food is dry and hard, it 

 requires much mastication and the saliva is secreted in abundance ; when 

 it is soft and moist, a smaller quantity of the secretion is poured out ; 

 and finally, food taken in a fluid form, as soup or milk, or reduced to 

 powder and moistened with a large quantity of water, is not mixed at 

 all with saliva, but passes at once into the stomach. 



The action of human saliva which converts boiled starch into sugar, 

 would seem to indicate a further connection with the digestive process. 

 This action will sometimes take place with great promptness in an arti- 

 ficial mixture of starch and saliva. Traces of glucose may be detected 

 in such a mixture in one minute after the two substances have been 

 brought in contact; and starch paste, introduced into the mouth, if 

 already at the temperature of 38 C., will yield traces of sugar at the 

 end of half a minute. Its rapidity, nevertheless, as noticed by Leh- 

 mann, varies much at different times. It is frequently impossible, even 

 with the mixture kept steadily at the temperature of 38 C., to find 

 evyience of sugar under five, ten, or fifteen minutes ; a difference prob- 

 ably dependent on the varying constitution of the saliva. 



Notwithstanding, furthermore, the occasional rapidity of this action, 

 it is not, on the whole, a very efficient one in regard to quantity ; that 

 is, only a small portion of the starch is converted into glucose within 

 a given time, the greater part remaining unchanged. This is proved 

 by the fact that such a mixture will exhibit the reaction of starch with 

 iodine long after Fehling's test has shown the existence of glucose. If 

 a solution of boiled starch, in the proportion of 3 parts of starch to 100 

 parts of water, be mixed with one-third its volume of fresh human 

 saliva, and placed in the water-bath at the temperature of 38 C., it 

 will often give, in one minute, a prompt sugar-reaction with Fehling's 

 test ; but it also contains, at the same time, an abundance of unaltered 

 starch. Even at the end of an hour, according to our own observa- 

 tions, the starch is far from being entirely converted, and the mixture 

 will still give a strong purple-blue color on the addition of iodine. 

 The same persistence of starch in considerable proportion may be seen 

 when the mixture is retained in the mouth. If a thin paste of boiled 

 starch, containing no traces of sugar, be taken into the mouth and 

 thoroughly mixed with the buccal secretions, it will often, as above 

 mentioned, begin to show the reaction of glucose in half a minute ; 



