ELEMENTARY PHYSIOLOGY. 
Saliva is a turbid frothy fluid secreted by the salivary glands 
■and various smaller glands in the lining membrane of the mouth. 
The active constituent of saliva is a ferment called ptyalin. The 
functions of saliva are two-fold: — 
(1) Mechanical, to moisten the mouth and gullet, and so assist 
in chewing and swallowing. 
(2) Chemical, by reason of the ferment ptyalin to convert the 
starchy matters (carbohydrates) into sugars. 
The flow of saliva is stimulated by the act of chewing and by 
the sight of food. 
The food having been masticated is collected on the surface of 
the tongue by the action of various muscles ; the tongue is pressed 
against the palate and the bolus of food pressed backwards to the 
root of the tongue where it passes between the pillars of the fauces 
into the pharynx. The muscles of the pharynx contract and the 
food is forced into the gullet, which, by a slow wave-like contraction, 
sends the food to the stomach. During the passage of the food 
over the larynx the posterior cartilages are approximated, and the 
tip of the anterior cartilage applied to them forming a triradiate 
fissure through which food cannot pass. Before food reaches the 
stomach this organ is collapsed ; the mucous membrane is pale in 
colour and thrown into ridges ; there is little movement and the 
secretion is scanty, only a little mucus being found covering the lining 
membrane. When food arrives, however, the blood vessels dilate, a 
secretion is poured out and churning movements begin. 
The gastric juice is secreted by glands situated in the mucous 
membrane of the stomach. It is a clear watery fluid containing 
hydrochloric acid, some organic salts and two ferments, one a protein- 
splitting ferment, pepsin, and the other a milk-curdling ferment, 
rennin. The pepsin acts on the proteins, converting them eventually 
into their simplest form, peptones. 
On fats the gastric juice has no action further than dissolving 
and digesting their protein covering and thus setting them free. 
The gastric juice is also antiseptic by reason of the free 
hydrochloric acid it contains. 
After gastric digestion has progressed for some time the partly 
digested food is passed on to the intestine where the process is 
completed. The stomach plays a very small part in the absorption 
of food, sugar and peptones only being taken up from it. 
In the intestine the food, which is now in a semi-fluid puitaceous 
condition and called chyme, meets with three different secretions, 
the bile, pancreatic juice and intestinal secretion. 
(i) Bile is the secretion of the liver, and its function is to act 
as a solvent of fats and assist in their digestion and absorption. 
