These terms do not, however, occur in any extant Hippocratic writings, unless, as seems possible to me, the latter term περητηρίῳ be a _var. lect._ for the obscure word τρυγλητηρίῳ applied to τρύπανον in ii. 470 in the description of trephining a hole through a rib to drain an empyema. Galen held the trephine in little esteem. It must have been difficult to manufacture a satisfactory instrument of bronze. In x. 448 he says: 'Some people, shall I call them rather cautious or rather timid, have used trephines' (χοινικίσιν); and Paul, in a passage I have already quoted, says: 'The mode of operating with saws and trephines is condemned by moderns as a bad one.' The term χοινικίς is derived from χοινίκη and χνόη, the nave of a wheel. The Latin term for the trephine, _modiolus_, has the same meaning. Celsus graphically describes the trephine and the method of its application. From him we learn how the ancients solved the problem of the centre-pin, which is necessary until the toothed portion has begun to bite. In modern trephines this difficulty is got over by withdrawing the pin up the centre of the shaft. In mediaeval trephines it was solved by providing two instruments, a male and a female, the male with centre-pin being used till a circular track had been cut by the toothed ring, the female without pin being then used. In the time of Celsus the centre-pin was removable, being taken out after the instrument had begun to bite. From Celsus too we learn that the trephine was driven by a thong. Celsus and Hippocrates both remark that, as in the case of the drill, it is necessary to dip the trephine in cold water at intervals in order to cool it, lest heat sufficient to injure the surrounding bone be generated. The thong manipulated by a bow would seem to be the method most applicable to an instrument like the trephine, which has a large boring radius, as slower motion is more easily produced by this arrangement than by one consisting of a cross-piece with thongs. Celsus says: