3i6 CHEMISTRY OF THE DIGESTIVE PROCESSES. 
a gland which lias been minced up and allowed to stand for a few hours (it 
may be in nearly all cases made faintly acid with very dilute acetic acid to set 
free the zymogen as enzyme). Such a minced-up gland is rubbed up in a 
mortar with some clean sand, taken up with glycerin, shaken up with more 
glycerin (10-20 parts to 1 part of gland), and allowed to stand so until 
required ; the process of extraction is very slow, and requires from seven to 
fourteen days. In the case of gastric mucous membrane, 1 part per 1000 of 
hydrochloric acid may be added to the glycerin. 
There are many modifications of the process. v. Wittich recommends 
digesting the minced gland (or mucous membrane) twenty-four hours in alcohol, 
drying after this in the air, sifting the powder through gauze to remove 
coarser fragments of tissue, and extracting with glycerin. It is often recom- 
mended to filter the extract after seven to fourteen days, but this is unnecessary, 
as the tissue neither decomposes nor becomes digested in the glycerin, and 
the extract improves on keeping in contact with the tissue. The enzyme 
accompanied by proteid may be precipitated from a glycerin extract by the 
addition of absolute alcohol, and so a purer extract be obtained. 
Chemical nature of enzymes. — The failure of all attempts to isolate 
pure enzymes necessarily deprives us of the possession of any certain 
knowledge of the chemical nature of these substances. Analyses of the 
purer preparations of the enzymes give figures ajiproximating to those 
obtained with the various proteids ; but whether or not this is due to 
admixture with proteid it is at present impossible to say. The behaviour 
of Brucke's " pure " pepsin solution goes against the supposition that this 
enzyme is a proteid. This solution did not give the proteid reactions, 
and was not precipitated by any of the proteid precipitants, save neutral 
and basic lead acetates and platinic chloride. These results are confirmed 
by Simdberg, 1 who succeeded in preparing a still more proteid-free solu- 
tion, which did not even react to these reagents, and was only precipi- 
tated as a slight, pure white, flocculent precipitate, on adding five to six 
times its volume of absolute alcohol and allowing to stand, and yet was 
exceedingly active in digesting fibrin. The amount of this precipitate 
was much too small for analysis, and it could only be shown that it was 
nitrogenous, and contained a certain amount of ash. This is not quite 
conclusive against the proteid nature of the active substance, since, as 
Simdberg argues, the physiological test by digestion may be much more 
delicate than any of the purely chemical tests. Still, the fact that it 
was totally unaffected by tannic acid and precipitated by alcohol has 
some weight against the substance being proteid in nature ; since tannic 
acid wdll show 1 part of ordinary proteid in 100,000, 2 and alcohol is by 
no means so delicate a proteid test. It is most probable, then, that 
pepsin is not a proteid ; and it will subsequently be seen in the descrip- 
tion of the other enzymes that most of these have been obtained in 
forms which do not yield all the proteid reactions. 
The enzymes are soluble in water, from which they are precipitable 
by saturation with ammonium sulphate or by adding excess of alcohol. 3 
Most of them are unalterable, or very slowly alterable in contact with 
alcohol, but pepsin is an exception, being attacked and rendered inactive 
if left long in contact. The enzymes are commonly said to be soluble 
1 Ztschr.f. physiol. Chan., Strassburg, 1885, Bd. ix. S. 319. See also under "Ptyalin." 
- Hofmeister, Ztschr. f. physiol. C'hem., Strassburg, 1878-9, Bd. ii. S. 292. 
3 These may only be particular cases of their general mechanical precipitation, whenever 
a precipitate is caused. 
