What is a " perambulating Skeleton"?

This would refer to the physical descrition of the dogs as walking skeletons. 

"As it was with Buck, so was it with his mates. They were perambulating skeletons. There were seven all together, including him. In their very great misery they had become insensible to the bite of the lash or the bruise of the club. The pain of the beating was dull and distant, just as the things their eyes saw and their ears heard seemed dull and distant. They were not half living, or quarter living. They were simply so many bags of bones in which sparks of life fluttered faintly."