> ANATOMY AND PHYSIOLOGY. 181 
curved form of the scabbard gives the barbs a certain 
amount of elasticity, without which such delicate 
instruments could scarcely be driven into any hard 
substance, while the fact of its tapering keeps the 
barbs close together at their pomts. Within the - 
body, where they no longer need the protection of 
the scabbard, the barbs leave the groove and, still 
diverging from each other, bend round either side of 
the muscular bulb till they end by being articulated 
at right angles to a strong horny limb or lever, which 
gives attachment to powerful muscles. By a rapid 
alternate movement of these levers the barbs are 
driven into the wound, the teeth of one barb acting 
as a fulcrum for the other. The two channelled sur- 
faces on the inner side of the barbs are pressed firmly 
against each other, so as to form a closed tube between 
them, down which, with all the force the muscular 
walls of the poison bag can exert, the venom is injected 
quite to the bottom of the wound. 
Fig. 11.—Diagram of the mechan- L 
ism of the sting. 
a, one of the barbs passing in a 
the guide, d, to the lever, c, 
which works at the same ad 
time on the feeler, 0. 
a 
The poison bag, which is readily recognizable as a 
hard roundish white body, is a strong hollow muscle 
made in four segments, like a foot-ball, the fibres of 
each segment meeting those of the adjoiming ones at 
an acute angle. At one end of this bag, two long 
glands may be observed, which secrete the poison. 
From the opposite end issues a long, strong duct, 
which conveys the poison from the interior of the bag 
