CHANGES FOOD UNDERGOES IN THE DIGESTIVE TRACT. 311 
but is most probably secreted by the ovoid or parietal 
cells, as the spheroidal cells in the centre of the glands do 
not give an acid reaction, while the ovoid ones in the neck or 
orifices of the glands are distinctly so. This view is sup¬ 
ported by the alkilinity of the gland cells at the pyloric end 
of the stomach, where the ovoid cells are not found. 
The gastric juice acts upon the albuminous bodies, which 
being colloid cannot diffuse through animal membranes until 
so acted upon, when they are converted into peptones. It 
has no action on starch, sugar or fats. It simply dissolves 
the connective tissue binding the fat vesicles together and 
sets the fats free, which become liquefied by the animal 
temperature. 
Proteid substances acted upon swell up, become semi¬ 
transparent, and then dissolve. There are several different 
bodies included under the peptones, as metapeptone, parapep- 
tone, dyspeptone, etc., which closely resemble one another in 
properties but differ from albumen (i) by readily diffusing 
through animal membranes ; (2) not coagulated by heat or 
nitric acid; (3) not precipitated by acetic acid and ferrocya- 
nide of potassium ; (4) giving a pink color on the addition of 
caustic potash. 
They resemble albumen in being precipitated (1) by 
tannic acid ; (2) by lead acetate. 
The gastric juice contains (1) pepsin ; (2) a curdling fer¬ 
ment ; (3) hydrochloric acid (free); (4) mucin ; (5) salts and 
water. 
Pepsin plays the part of a ferment, but not acting on the 
proteids until the hydrochloric acid has reduced them to acid 
albumen, metapepone, parapeptone, etc. They are then 
changed into a soluble, diffusible and uncoagulable product- 
peptone. 
Pepsin itself cannot convert albumen into peptone. 
Albumen may without the presence of pepsin give rise to 
peptone if treated with strong acids, alkalies, boiling under 
high pressure, putrefaction and other fermentative actions. 
Pepsin occurs in small quantities and is not destroyed in the 
act of digestion. 
