He had plenty of nerve.

He had plenty of experience along his chosen line of endeavor.
But, while a crook may control his nerve, he cannot make it phlegmatic or steady.

Always, he must be conscious of holding it in check, as a clever driver checks and steadies and keeps in subjection a plunging horse.

Let the vigilance slacken, and there is a runaway.
Now this particular marauder had long ago keyed his nerve to the chance of interruption from some gun-brandishing householder; and to the possible pursuit of police; and to the need of fighting or of fleeing.
But all his preparations had not taken into account this newest emergency.

He had not steeled himself to watch unmoved the gliding away of a treasure-satchel, apparently moving of its own will; nor the shimmer of two greenish sparks in the air just above it.