THE DIGESTION AND UTILIZATION OF CRUDE FIBRE          157

digestive system of carnivores is not naturally adapted for plant food, a dog may
develop an intestinal flora which makes possible a certain amount of digestion of
fibre. The house dog is fed as an omnivorous animal and often with much plant
food so that, with this food, it will also ingest cellulose splitting bacteria. Bacterial
fermentation in this case will probably attack chiefly the more delicate vegetable
celluloses which are also digestible by man. But the fibre of potatoes and cereal
grains were found to be largely digestible in experiments of Lossl at this Institute
on fox terriers, although there were wide variations in different experiments. Of
the Weende fibre, the dogs digested; from barley meal 7.67—33.53 ; potato flakes,
15.37-23.07; boiled potatoes, 3.27—86.83 ; rye meal, 14.54-24.28 ; wheat meal,
2.92—87.52 and rice, 25.24—80.05 per cent.

These wide variations in fibre digestion by the dog appear understandable in the
light of the possible differences in bacterial flora in the intestine and the similar
observations on the wide individual differences in fibre digestion in man, In man,
also, the alimentary tract normally digests cellulose in different degrees depending
on its origin, age and hardness; sometimes digestion may be almost complete.

On account of the fact that most investigations have been made on a limited
number of subject with only single experiments, and of the large range of variation,
it is impossible to give figures for the average digestibility by man of fibre in
different foodstuffs. Weiske (143) was the first to study fibre digestion in man,
and he found in two subjects, on pre- and post-experimental diet free from cellulose,
digestibility coefficients of 47.3 and 62.7 per cent. for carrots, celery and cabbage.
v. Knieriem (69) found 25.32 per cent. digestibility for fibre of lettuce. Lohrisch
(76) on the basis of a few 1-2 day experiments on 3 subjects, gives the following
fibre digestibilities . white cabbage, 100 ; carrots, 95 ; spinach, 90 ; bread, 59-85 ;
split peas, 45 per cent. Rubner carried out numerous single experiments on human
subjects in his studies of the digestibility of plant foods, using different kinds of
vegetables, fruits and bread. Of the chemical components of the cell membrane,
he found in some kinds of fruit and vegetables, that cellulose was 79.06, pentosan,
81.81 and residue, 82.86 per cent. digestible. For different types of bread the
corresponding figures were 32.81, 45.46 and 33.26 per cent. The cell membrane of
potatoes was particularly easily digested, a characteristic which justifies their
wide use as a foodstuff (108).

In the study of the digestion of fibre in man, microscopic examination of the
faeces is of particular importance. It is found that the delicate membranes of, for
instance, savoy cabbage or carrots (Rubner, 102) are completely digested ; those of
spinach, with the exception of the lignified fibres of the spiral vessels (35, 47), and
those of potatoes also to a great extent (1,129). Well lignified parts and cork are
not affected (101).