DIGESTION OF FOOD. 307 
ling’s solution (copper sulphate in an excess of sodium hydrate, 
the sugar reducing the cupric hydrate to cuprous oxide on 
boiling). 
2. At this early stage starch may still be detected by the 
blue color it gives with iodine; but later, instead of a blue, a 
purple or red may appear, indicating the presence of deatrin, 
which may be regarded as a product intermediate between 
starch and sugar. 
3. The longer the process continues, the more sugar and the 
less starch or dextrin to be detected; but, inasmuch as the 
quantity of sugar at the end of the process does not exactly 
correspond with the original quantity of starch, even when no 
starch or dextrin is to be found, it is believed that other bodies 
are formed. One of these is achroodextrin, which does not give 
a color reaction with iodine. 
The sugars formed are: (a) Dextrose. (b) Maltose, which 
has less reducing power over solutions of copper salts, a more 
pronounced rotatory action on light, etc. 
It is found that the digestive action of saliva, as in the 
above-described experiment, will be retarded or arrested if the 
sugar is allowed to accumulate in large quantity. That diges- 
tion in the mouth is substantially the same as that just de- 
scribed can be easily shown by holding a solution of starch in 
the mouth for a few seconds, and then testing it for sugar, 
when it will be invariably found. 
While salivary digestion is not impossible in a neutral 
medium, it is arrested in an acid one even of no great strength 
(less than one per cent), and goes on best in a feebly alkaline 
medium, which is the condition normally in the mouth. Though 
a temperature about equal to that of the body is best adapted 
for salivary digestion, it will proceed, we have ourselves found, 
at a higher temperature than digestion by any other of the 
juices, so far as man is concerned—a fact to be connected, in all 
probability, with his habit for ages of taking very warm 
fluids into the mouth. 
\. The active principle of saliva is ptyalin, a nitrogenous, body 
which is assumed to exist, for it has never been perfectly iso- 
lated. It belongs to the class of unorganized ferments, the 
properties of which have been already referred to before (page 
160). 
eer of the Secretion of the Different Glands.—Parotid 
saliva is in man not a viscid fluid, but clear and limpid, con- 
taining very little mucin, Submaxillary saliva in most animals 
