HOW FOOD IS DIGESTED 73 
salts by the kidneys and by perspiration; water by the 
organs of excretion. Fat and sugar when oxidized yield 
simply water and carbonic acid and are excreted as such 
products. The undigested part of the food is voided as 
solid excrement. 
The powers of digestion differ with the several species 
of animals. Thus hogs are 
not capable of living on hay 
and similar roughage because 
their single small stomach 
does not permit them to digest 
a sufficient proportion of such 
material. A hog will not 
grow fat on grass alone, while 
a steer may. Cattle and sheep 
with their four stomachs, 
have the power of efficiently 
changing grass into food, 
clothing and labor. It is this 
power which makes them 
useful to man. Horses have 
RESPIRATORY SYSTEM OF HORSE 
1. Cranial cavity; 2, guttural 
pouch; 3, nasal cavity; 4, tongue; 
5, pharyngeal cavity; 6, cavity of 
larynx; 7, epiglottis; 8, trachea; 9, 
cesophagus; 10, section of left 
bronchus; 11, ramifications of right 
bronchus; 12, right lung; 13, left 
lung; 14, sternum; 15, ribs; 16, 
heart; 17, posterior aorta; 18, ante- 
rior aorta. 
the ability of digesting coarse materials in a fairly high 
degree. 
When severe work, however, is required horses 
must be fed on concentrated and easily digested food for 
the best results. 
