Sentence:  A cord, in passing round a pulley, lies in a groove, sometimes called the <WSD>"gorge</WSD>" of the pulley; if the object of the pulley is merely to support, guide, or strain the cord, the <WSD>gorge</WSD> may be considerably wider than the cord; if the pulley is to drive or to be driven by the cord, so as to transmit motive power, the <WSD>gorge</WSD> must in general fit the cord closely, or even be of a triangular shape, so as to hold it tight. 
Meaning of the word marked with <WSD> tags:  The groove of a pulley.