DIGESTION OF FOOD. 291 
‘We must thus recognize something more in an animal than 
merely the mechanical and chemical processes which suffice to 
accomplish digestion in the laboratory. A food must be not 
only sufficient from the chemical and physical point of view, 
but be capable of being acted on by the digestive juices, and 
of such a nature as to suit the particular animal that eats it. 
To illustrate, bones may be masticated and. readily digested 
by a hyena, but not by an ox or by man, though they meet the 
conditions of a food in containing all the requisite constituents. 
Further, the food that one man digests readily is scarcely digesti- 
ble at all by another; and it is within the experience of every 
one that a frequent change of diet is absolutely necessary. 
Since all mammals, for a considerable period of their exist- 
ence, feed upon milk exclusively, this must represent a perfect 
or typical food. It will be worth while to examine the compo- 
sition of milk. The various substances composing it, and their 
relative proportions for different animals, may be seen from the 
following table, which is based on a total of 1,000 parts: 
CONSTITUENTS. Human. ‘Cow. Goat. ‘Ass. 
Waterncceaceees araeeals 889-08 857-05 86358 910:24 
Cas@inl nxcccecwccce aa ve ' 48-28 33°60 ? : 
Albumin....0 1202.02! co | 5°76 12:99 2018 
Buttetss css sansa reves 26°66 43:05 43:57 12:56 
Milk-sugar ............. 43°64 40°37 40°04 57-02 
Salts: sccc.% sxe ncnnes, 1:38 5-48 6:22 
Total solids......... 110°92 14295 186-42 + 89-76 
The fact that human milk is poorer in proteids and fats 
especially is of practical importance, for, when cow’s milk is sub- 
stituted in the feeding of infants, it should be diluted, and sugar 
and cream added if the normal proportions of mother’s milk 
are to be retained. 
1. The proteids of milk are: 
(a.) An albumin very like serum-albumin. 
(b.) Casein, normally in suspension, in the form of extremely 
minute particles, which contributes to the opacity of milk. 
It can be removed by filtration through porcelain; and pre- 
cipitated or coagulated by acids and by rennet, an extract of 
the mucous membrane of the calf’s stomach. After this coagu- 
lation, whey, a fluid more or less clear, separates, which con- 
tains the salts and sugar of milk and most of the water. Much 
of the fat is entangled with the casein. 
