It was Tumbu's low growl as he realised that some one wanted to touch his old master and that his old master did not want to be touched.
"At him, Tumbu! At him, good dog!" The words came to Roy in a flash, and like a flash the great, powerful dog leaped forward, his fur a-bristle, his white teeth gleaming, and the next instant, taken by the suddenness of the attack, the sentry lay on his back half stunned by the fall, while Tumbu, on the top of him, checked even a cry by a clutch at his throat.

A soft clutch so far; but one that would tear through flesh if needful.
Roy was on his knees beside the fallen man.
"Hist! not a sound or the dog shall kill you.

He can.

Give me the keys.
I want to get out of the gate! The keys, do you hear ?" The sentry tried to struggle, but warned by the weight of the dog on his breast and those sharp teeth ready to close upon his throat, murmured hoarsely, "It is only barred, but the bolts are difficult.

If you will let me get up and call off your dog----" But Roy took no heed of his words.