810 ANIMAL PHYSIOLOGY. 
power of the gastric juice is greater than can be accounted 
for by the presence of the acid it contains merely, and it has 
a marked antiseptic action. 
Digestive processes may be conducted out of the body in a 
very simple manner, which the student may carry out for 
himself; To illustrate by the case of gastric digestion: The 
mucous membrane is to be removed from a pig’s stomach 
after its surface has been washed clean, but not too thoroughly, 
chopped up fine, and divided into two parts. On one half pour 
water that shall contain ‘2 per cent hydrochloric acid (made 
by adding 4 to 6 cc. commercial acid to 1,000 cc. water). This 
will extract the pepsin, and may be used as the menstruum in 
which the substance to be digested is placed. The best is fresh 
fibrin whipped from blood recently shed. 
Since the fluid thus prepared will contain traces of peptone 
from the digestion of the mucous membrane, it is in some 
respects better to use a glycerine extract of the same. This is 
made by adding some of the best glycerine to the chopped-up 
mucous membrane of the stomach of a pig, etc., well dried with 
bibulous paper, letting the whole stand for dight to ten days, 
filtering through cotton, and then through coarse filter-paper. 
It will be nearly colorless, clear, and powerful, a few drops suf- 
ficing for the work of digesting a little fibrin when added to 
some two per cent hydrochloric acid. 
Digestion goes on best at about 40° C., but will proceed in 
the cold if the tube in which the materials have been placed is 
frequently shaken. It is best to place the test-tube containing 
them in a beaker of water kept at about blood-heat. Soon the 
fibrin begins to swell and also to melt away. 
After fifteen to twenty minutes, if a little of the fluid in the 
tube be removed and filtered, and to the filtrate added carefully 
to neutralization dilute alkali, a precipitate, insoluble in water 
but soluble in excess of alkali (or acid), is thrown down. This 
is in most respects like acid-albumen, but has been called para- 
peptone. The longer digestion proceeds, the less is there of 
this and the more of another substance, peptone, so that the 
former is to be regarded as an intermediate product. Peptone 
is distinguished from albuminous bodies or proteids by—1. 
Not being coagulable from its aqueous solutions on boiling. 
2. Diffusing more readily through animal membranes. 3. Not 
being precipitated by a number of reagents that usually act 
on proteids. 
In artificial digestion it is noticeable that much more fibrin 
