46 
CHEMISTRY OF PEPSIN. 
most recently experimented in this direction, 100 grammes 
of canine gastric juice will dissolve and digest forty grammes 
of dried fibrin—a widely different and, 1 think, quite irre¬ 
concilable result. 
Nature of Pepsin. —Pepsin, the organic constituent of the 
gastric juice, was first isolated and examined by Wasmann. 
He found that if the mucous membrane of a stomach were 
treated with cold water and the infusion evaporated to dry¬ 
ness, there remained a viscid, brownish mass, having the odour 
of glue. The properties of a solution of this substance may 
be thus shortly stated. 
It is precipitated by the addition of alcohol, tannin, or 
acetate of lead, forming in the latter case a compound of 
tolerably definite constitution—peptate of lead. It possesses 
the power of digesting nitrogenized substances if they 
are subjected to its action under fit conditions of heat and 
motion. 
If the peptate of lead be decomposed by sulphide of 
hydrogen, a solution of pure pepsin is obtained. 
This solution, which is neutral, is of itself incapable of 
effecting digestion ; but if a small quantity of an acid, as lactic 
or hydrochloric, be added, solution proceeds rapidly. The 
pepsin seems, therefore, to dispose the acid to dissolve the 
substance, somewhat simulating, in fact, the action of a 
ferment. Indeed, the ferment theory would seem to be 
borne out by the action of pepsin upon grape sugar, which it 
converts into lactic acid. Here I must not omit to allude, in 
passing, to the manner in which the starchy portions of the 
food are digested. The action of the ptyalin in the saliva 
first converts the starch into grape sugar, which is capable of 
being assimilated without digestion, and by the further 
action of pepsin upon the grape sugar a sufficiency of 
lactic acid is produced to carry on the digestion of nitro¬ 
genized food. The changes which take place may be thus 
represented : 
One equivalent of starch == C l2 H n O ]] becomes 
One equivalent of glucose = C 12 H ]2 O n which is finally 
changed into 
Two equivalents of lactic acid — 2(C 6 H 6 O fi ). 
Artificial Pepsin .—The difficulty of conveniently obtaining 
the gastric juice of animals for medicinal purposes, and the 
disgust which many patients not unnaturally felt at its use, 
soon led to its being altogether discarded, and it is only very 
