One great encouragement in cases requiring this operation is the fact that in every successive pregnancy the uterus is more easily excited to premature action; and in some cases where it has been induced several times, it has at length, as it were, got so completely into the habit of retaining its contents only up to a certain period, that labour has come on spontaneously exactly at the time at which in the former pregnancies it had been artificially induced.[103] We have already alluded to this circumstance in the chapter on PREMATURE EXPULSION OF THE FOETUS.[104] CHAPTER V. PERFORATION. _Variety of perforators.--Indications.--Mode of operating.-- Extraction.--Crotchet.--Embryulcia._ The perforation is that operation "where we make an opening into the cranial cavity, and, by allowing the brain to escape, thus diminish the bulk of the head." (_Obstetric Memoranda._) Perforation is one of the most ancient operations in midwifery, for in former times it was the only means of artificially delivering the child when the head presented: hence we find that from the age of Hippocrates down to the last century, midwifery instruments almost entirely consisted of knives or lancets for piercing the foetal head, and blunt or sharp hooks for extracting or dismembering the child. Thus Hippocrates, Celsus, and Albucasis, and others, have described a variety of such instruments and given full directions for their use. _Variety of perforators._ No instrument has been so greatly modified or has appeared under such different forms as the perforator; but it is not our object to enter into any detailed account of its history, for it would not, like that of the forceps, lead to any useful information; we shall, therefore, content ourselves with mentioning those few which have been in general use during the last century. They are chiefly of the scissor kind; the two most commonly known are the perforators of Dr. Smellie and M. Levret: the former are merely strong long-handled scissors, the backs of the blade being neither exactly sharp nor blunt,[105] and furnished each with a projecting shoulder or rest to prevent them from entering too far. Levret's perforator, which is extensively used in this country under the name of Dr. Denman's perforator, and which was originally invented by Bing, of Copenhagen, is also formed like scissors, but has its cutting edges outside; the blades are also furnished with rests or shoulders like the Smellie perforator. [Illustration: Naegelé's perforator.]