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86 DENTAL MEDICINE.
acid generated in the alimentary canal, thus becoming converted
into a purgative salt, as magnesia, for example. Others expend
their principal force upon the rectum and large intestines, and for
this reason are advantageously employed in affections of the
lower bowels, and the uterus and its appendages, such as aloes,
for example. Cathartics should always be given on an emptv
stomach. If administered immediately after a full meal, they
arrest the digestive process, are liable to cause nausea, if not
vomiting, and do not evacuate the bowels with the same certainty
or effect.
When simple costiveness is to be removed, the cathartic may
be administered in the evening, and, if not sufficient, the dose
can be repeated in the morning and at regular intervals through
the day, until the effect is produced.
In administering cathartics, the excitement is to be continued
till the requisite action is induced, yet not sufficient to prove an
irritant. But in the treatment of many diseases, it is preferable
to pui^e through the day, in order that the sleep may not be dis-
turbed at night. The operation of cathartics may be ver\- much
accelerated by the free use of diluent drinks, such as gruel, bar-
ley water, etc., etc. By combining these remedies, the action of
many of them is modified and controlled; and some of the more
powerful mav be made to operate mildly and certainly by uniting
small quantities of several of them in the same dose.
The addition of an emetic substance, such as ipecacuanha, or
tartar emetic, gives activity to the combination, while it modifies
the harshness of the powerful cathartics.
The operation of a cathartic may often be promoted by judi-
cious venesection, and if there is spasm of the intestines, opium
may be advantageously added to the cathartic.
Cathanics act not only upon the bowels, but upon distant
parts, as every portion of the organism is capable of being im-
pressed by them. Cathartics are divided, according to the in-
tensity of their operation, into laxatives^ purgatives and drastics^
to which may also be added enemata. Laxatives gentlv stimulate
the mucous coat of the intestines, and hence they are well
adapted for cases in which the sole indication is to unload the