262 
THE ART OF REARING 
and earthy substances, and others, that may be 
contained in various proportions in all vegetables. 
These substances are generally extraneous to the 
necessities of its economy ; for which reason, had 
nature not provided the animal with the means 
of daily expelling, the oppressed animal would 
decay and perish. 
Nature furnishes three resources for this pur- 
pose, — cutaneous transpiration, pulmonary tran- 
spiration, and urinary evacuation. I do not cal- 
culate here the solid and excremental substances 
which are voided by the intestinal tube. 
It often occurs that one of these means supplies 
the others, as in the frequent cases of men who 
perspire much and lose little by urinary evacua- 
tions. 
It must also be noted, that transpiration cannot 
exist without the contact of air, and that there is 
great analogy between the constituent principle 
of urine and transpiration. 
The health of animals requires that they should 
expel, by means of the excreting organs, the super- 
abundant liquid and extraneous substances which 
may have been introduced into their organization 
by nutrition. 
If the excretions are impeded, the animal will 
be attacked by dangerous diseases, such as we see 
occur in men from the same general cause. 
The silk-worms growing in proportion to the 
