DIGESTION OF FOOD. 811 
or other proteid matter will be dissolved if it be finely divided 
and frequently shaken up, so that a greater surface is exposed 
to the digestive fluid. 
The exact nature of the process by which proteid is changed 
to peptone is not certainly known. 
Since starch on the addition of water becomes sugar (CeHio 
O;+H;,O = C.H..0.), and since peptones have been formed 
through the action of dilute acid at a high temperature or by 
superheated water alone, it is possible that the digestion of 
both starch and proteids may be a hydration ;. but we do not 
know that it is such. 
As already explained, milk is curdled by an extract of the 
stomach (rennet) ; and this can take place in the absence of all 
acids or anything else that might be suspected except the real 
cause; there seems to be no doubt that there is a distinct fer- 
ment which produces the coagulation of milk which results 
from the precipitation of its casein. 
The activity of the gastric juice, and all extracts of the mu- 
cous membrane of the stomach, on proteids, is due to pepsin, a 
nitrogenous body, but not a proteid. 
Like other ferments, the conditions under which it is effect- 
ive are well defined. It will not act in an alkaline medium at 
all, and if kept long in such it is destroyed. In a neutral me- 
dium its power is suspended but not destroyed. Digestion will 
go on, though less perfectly, in the presence of certain other 
acids than hydrochloric. As with all digestive ferments, the 
activity of pepsin is wholly destroyed by boiling. 
When a large quantity of cane-sugar is taken into the 
stomach, an excess of mucus is poured out which converts it, 
presumably by means of a special ferment, into dextrose. 
Bile—The composition of human bile is stated in the fol- 
lowing table: 
WAt6r pec se nine peewee aoe teshee Scola 82-90 per cent. 
Bilessal teins ose see ee ae otaviad age dees 611 “ 
Fats and. s0apsiccisccccinsaceee sees aoe 2 ee Ne 
Cholesterin: + ssivieses sence wiv sae sess 04 “os 
Lecithinweiws: #200 agreed dees ween oeide iG as 
MIS eNa hee ea Daa seaauare snake BABE) ORY OE: 
ING ocaesisaane Samet es nat a santa chin a ebeiens O61 “ “ 
The color of the bile of man is a rich golden yellow. When 
it contains much mucus, as is the case when it remains long in 
the gall-bladder, it is ropy, though usually clear. Bile may 
.contain small quantities of iron, manganese, and copper, the 
