Sentence:  Dr. Prunesquallor had circled around Steerpike with his head drawn back so that his cervical vertebrae rested against the near wall of his high collar, and a plumbless <WSD>abysm</WSD> yawned between his Adam's apple and his pearl stud. 
Meaning of the word marked with <WSD> tags:  An abyss; a gulf, a chasm, a very deep hole. (from late 15th century)<ref name=SOED/>