Sentence:  He seemed tired, and the Rat let him rest unquestioned, understanding something of what was in his thoughts; knowing, too, the value all animals attach at times to mere silent companionship, when the weary muscles <WSD>slacken</WSD> and the mind marks time. 
Meaning of the word marked with <WSD> tags:  To gradually decrease in intensity or tautness; to become slack.