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This treatment, too, must be different in the case of different foods 
of the same class. The treatment which removes the cellulose 
may injure the contents of one but not of another, so that each 
food must be individually studied. Where these facts are totally 
neglected, as in the case of savages who consume coarse vege- 
table food in the raw condition, the abdomen commonly becomes 
permanently enlarged and distorted. 
In speaking of the indigestibility of the carbohydrate cover- 
ing to the vegetable cell contents, I have assumed them to be in 
the most favorable condition for digestion, that of consisting of 
pure cellulose, little or not at all thickened. 
A typical illustration of such a food is the potato, Yet in this 
most favorable example, digestion is so difficult that fatal results 
from indiscreet feeding are of the most common occurrence. The 
stomachs of infants which have died in convulsions are often found 
to contain pieces of undigested potato, usually imperfectly cooked, 
though often perfectly cooked, but eaten without the thorough 
mashing required to break up the mass, and partly,fat least, dis- 
integrate the cell-wails. 
We must now consider certain changes which occur in cell- 
walls which tend very greatly to increase the comparatively mod- 
erate degree of indigestibility illustrated in the potato. Among 
the various functions performed by the tissues of plants, that of 
affording mechanical strength, is highly important. This quality 
is secured by the thickening: of the cell-walls, sometimes to so 
great an extent that it fills up most, or even all, of the cell-cavity. 
This added material is almost always a different kind of substance 
from the original cellulose, and of a far less digestible character. 
For example, the surface-tissues of the plant must protect those 
within from the escape of any of their gases or fluids, except as 
naturally provided for by special openings ; also from the access 
of injurious fluids from without. Such a power is secured by 
the thickening of the cell-walls by substances called cutin and 
suberin, both highly impervious and highly insoluble. The sec- 
ond named is the substance which forms cork. The highly in- 
digestible character of such substances is apparent. The inner 
tissues, requiring strength and toughness, as in the formation of 
