ned from different
samples; and it has been ascertained that the elytra or wing-sheaths of
the insect, which alone are used in pharmacy, contain more of the active
principle than the soft parts taken together; but apparently
cantharidin is most abundant in the eggs and generative organs.

Cantharidin constitutes from 1/2 to 1% of cantharides. It has the
formula C10H14O4, and on hydrolysis is converted into cantharinic acid,
C10H14O5. It crystallizes in colourless plates and is readily soluble in
alcohol, ether, &c., but not in water. The British Pharmacopeia contains
a large number of preparations of cantharides, but the only one needing
special mention is the tincture, which is meant for internal
administration; the small dose is noteworthy, five minims being probably
the maximum for safety.

The external action of cantharides or cantharidin is extremely
characteristic. When it is applied to the skin there are no obvious
consequences for some hours. Thereafter the part becomes warm and
painful, owing to marked local vascular dilatation. This is the typical
_rubefacient_ action. Soon afterwards there is an accumulation under the
epidermis of a serum derived from the dilated blood-vessels. The
numerous small blisters or vesicles thus derived coalesce, forming a
large sac full of "blister-fluid." The drug is described as a
counter-irritant, though the explanation of this action is very
doubtful. Apparently there is an influence on the afferent nerves of the
part which causes a reflex contraction--some authors say dilatation--of
the vessels in the internal organs that are under the control of the
same segment of the nervous system as that supplying the area of skin
from which the exciting impulse comes. When applied in this fashion a
certain quantity of the cantharides is absorbed.

Taken internally in any but minute doses, the drug causes the most
severe gastro-intestinal irritation, the vomited and evacuated matters
containing blood, and the patient suffering agonizing pain and extreme
depression. The further characteristic symptoms are displayed in the
genito-urinary tract. The drug circulates in the blood in the form of an
albuminate and is slowly excreted by the kidneys. The effect of large
doses is to cause great pain in the renal region and urgent wish to
micturate. The urine is nevertheless small in amount and contains
albumen and blood owing to the local inflammation produced in the kidney
by the passage of the