This expression refers to a dog chewing endlessly on a bone. It picks it clean.

这个表达指的是狗没完没了地。得干干净净。

And dogs often like to chew bones.

而且狗经常喜欢。

It reminds him of his old German Shepherd gnawing on bones; he freezes.

这他想起了他的老德国牧羊犬；他僵住了。

And we know this because, in most of the big bone-crushers, their cheek teeth show distinctive marks -- the same marks that modern hyenas get by gnawing on bones.

我们之所以知道这一点， 是因为在大多数大型碎机中，它们的颊齿上都有明显的痕迹——与现代鬣狗时留下的痕迹相同。