CHANGES FOOD UNDERGOES IN THE DIGESTIVE TRACT. 309 
The flow of blood is secondary to the secretion called 
forth by stimulation, as after ligating- the vessels, an amount 
of saliva can be made to flow from the gland, which could 
not have been stored up in the cells prior to ligating. 
The activity of secretion and vascular dilatation is due to 
different nerves, as shown by atropia stopping the secretion 
of saliva, but not preventing the dilatation of the vessels, 
proving that its effect is restricted to a mechanism engaged 
exclusively in controlling the activity of the gland cells. 
The cells are constantly undergoing repairs and are never 
perfectly quiescent, but are diligently adding to their stock in 
hand, as a result of the intrinsic power of their protoplasmic 
contents. 
Saliva is a viscid, frothy, alkaline fluid. Specific gravity 
about ten hundred and five, and contains: (i) mucin, 
(2) phyalin, (3) albumen, (4) salts and water, also epithelial 
cells, mucus and salivary corpuscles. 
The mucus is obtained principally from the sub-maxillary, 
sub-lingual and small mucus glands. 
Among the salts is found potassium sulphocyanite, a con¬ 
stituent not found in the blood. 
The saliva moistens the particles of dry food and makes 
them adhere together, also facilitates deglutition by coating 
the bolus with a viscid fluid, and converts starch into grape 
sugar, which is due to a ferment called ptyalin. 
Ptyalin effects alterations on the substances on which it 
acts, without undergoing any perceptable change or diminu¬ 
tion itself. It exists in such small quantities that its presence 
can only be known by the effects it produces, and is most 
active at bodily temperature. Ptyalin acts on starch and is 
hence called amylolytic ferment. Its mode of converting 
starch into grape sugar is by uniting to the starch chemically 
one molecule of water. 
During this process, which takes a few minutes to com¬ 
plete, various stages can be detected. First, two substances are 
formed, which together are called dextrin. One easily pass¬ 
es into soluble sugar and give a red color with iodine, and is 
called erytho-dextrin. The other gives no color with iodine, 
