^'uNE 1, 1903.] Supplemen/ to tie " 
enaixielled saucepan over a clear fire till £et. Pile 
on the croutons, fcatter a little chopped pnigley 
ou each, nnd tcrve fit oi.'ce. 
PRINCIPLES OF KUTEITION A]SD KLIRI- 
TIVE VALUE OE FOOD. 
{Continued.) 
"We live rot upon what we eat, but upon wliat 
we digest," Food as we buy it in the market, 
or even as we eat it, is not usually in condition to 
be made into body structure or used as body fuel. 
It must first go through a series of chemical 
changes by what is called digestion, which prepare 
it to be absorbed, taken into the blood and lymph, 
and carried to the parts of the body where it is 
needed. Digestion takes place in the alimentary 
canal, partly in the stomrch, but more in the 
intestine. As the result, the useless portions are 
separated and rejected, while the parts which 
can serve for nutriment are changed into forms in 
which they can be absorbed, taken into the 
circulation, and utilized. 
The alterations which the food undergoes in 
digestion are brought about by substances called 
ferments, which are secreted by the digestive 
organs. The saliva in the mouth has the power of 
changing insoluble starclies into soluble sugar, but 
as the food stays in the mouth only a short time, 
there is generally little chance for such action. 
The saliva, however, helps to fit the food to be more 
easily worked on by th.e stomach. The gastric 
juice secrettd in tlie stomach acts upon the 
protein, fats, and carbchydrates. The action of 
all the fermei'.li is aided by the fine divis-ion of 
the food by chtwing and by the muscular cor.trac- 
t ns, the so so-called peristaltic action, of the 
s lomach and ii.testine. These latter motions help 
to mix the digestive juices and their feimeuts with 
th^ food. 
The poits tf the fcod which the digestive juices 
cannot dissolve, and which therefore escape diges- 
tion, are periodically givtn cff by the intestine. 
Such solid excreta or feces, ii:clude not only the 
particles of undigested fcod, but also the so-called 
metabolic products, i'c, residues of the digestive 
juices, bits of the lining of the alimtnlaiy 
canal, etc. 
The digested foid finds its way through the 
v\ alls of the alimentary cantil, ar.d in this passage 
undergoes nnuirkable chemical changes, as is 
teen when we compare the contents of the ali- 
meutaiy canal bef(;re they are absorbed and taken 
into the circulation, with the blood which has 
been made from the digested material. 'Wlien 
finally the bleed, suj plied w ith the nutiients 
of the digested food and fieighted with oxygen 
from tlie lungs, is pumped from the h.earl all 
over the body it is ready to furnish the organs 
and tissues with the materials and energy v\hich 
they r.ted for their peculiar fui cticus ; nt the 
SI. me time it carries away the v\ oste which ti e 
exercise ()f these functions has produced. It 
is a characteiistic of living body tissue that it 
can cheese tlie necessaiy mafirials trom the blood 
ai.d build them into its own structure, llow 
it does this is one of the mysteries of physic logy 
Tfoi>ical AgncultuvhV BVl 
Tlie body, ne we linve learned, has also tlie power 
of consuming not only the materials of the food 
but also paits of its €>wn structure for the pro- 
duction of muscular work, or heat, or to protect 
more important parts from cousumpi ion. How 
it does this is another my.-tery, still to be ex- 
plained. 
After the material has been thus assimilated and 
utilized the resulting waste products must be 
removed from the body. The chemical elements 
vvhich this waste contains are of course the same 
as those making up the stiucture of the body 
and the food — carbon, oxygen, hydrogen, nitrogen, 
calcium, phosphorus, sulpluii', etc. Most of the 
carbon and part of the oxygen are given off from 
the Jungs as caibon dioxid. Hydrogen unites 
with more oxyge'n to form warter, which is passed 
off in vapour from the lungs, in perspiration from 
the skin, and in urine from the kidneys. Almost 
all the jjitrogen is excreted in the urine. Waste 
mineral matters are given off to seme extent in 
the perspiration, but mniiily through the kidneys 
and intestines. 
The real nutritive value of a food, then, 
depends not simply on the proportions of nutri- 
ents which it contains, but also on the amount of 
those nutrents wliich can be made available to the 
body by digestion for building material and for 
fueh Part of the food eaten escapes digestion 
and is given off from the body in tlie feces. If 
we subtract the amount of this undigested residue 
from the total food the remainder will be the 
amount actually digested in the stomach and 
intestines, absorbed through tlieir walls and taken 
into the circulation. This differet.ce between the 
amounts eaten and those undigested represents 
the actual digestibility of food. A part of the 
food taken into the circulation, however, is later 
returned again to the alimentary canal mainly in 
the digestive juices that are needed for digesting 
the food. The material thus removed from cir- 
culation and returned to the elementary canal, 
which consists of so-called metabolic products, 
is excreted with the u> digested residue in the 
feces. The remainder of the food taken into the 
circulation represents the amount retained by the 
body for building material and for fuel. The 
difiereuce between the food which is absorbed 
and that which the bcdy secures, theiefore, is re- 
presented by the metaLolic producis. By the 
piesent methods of experimenting, however, the 
portion of the feces that consists of metabolic 
products cannot be satisfactorily distirgui^hed 
from the undigested residue. It is very difhculr, 
tb.eiefore, to determine the actual digestibility, 
but comparatively easy to estimate the apparent 
digestibility of food.* 
' It would be more exact tense fiiffercnt terms to 
citnote the cppaifnt oiigc s^tibilily of feed es dislin- 
^nitheo ficm ils aclnal digcstibilily. It has theitfore 
befu pici'ctf d to limit the use cf the term digestibility 
to actuEl cligcstibilily ai d cnilicy the teim avr-ilr bility 
ViV.cn airft'^t dipesrtibility, as cidiLaiiiy deteiniiLCil 
in nutiiticn ii vpElignticns!, isn.e-nt. iSec Ccnne client 
Stoiis Station Repent 16F9. p. G9.) 
t Willie m litivinicnt, The Pbysiolcgy of Digestion, 
Viitli Exvciinunls cn the Gf.stric Jnice. £d td. Bui- 
liijgtcu, Vt., ISil, 
