Great conceits are raised of the involution or membranous covering, commonly called the Silly-how, that sometimes is found about the heads of children upon their birth, and is therefore preserved with great care, not only as medical in diseases, but effectual in success, concerning the Infant and others; which is surely no more than a continued superstition. For hereof we read in the life of Antoninus delivered by Spartianus, that children are born sometimes with this natural cap, which Midwives were wont to sell unto credulous Lawyers, who had an opinion it advantaged their promotion.
But to speak strictly, the effect is natural, and thus to be conceived: Animal conceptions have three teguments, or membranous films which cover them in the womb, that is, the Corion, Amnios, and Allantois... Now about the time when the Infant breaketh these coverings, it sometime carrieth with it about the head a part of the Amnios or nearest coat; which, saith Spiegelius, either proceedeth from the toughness of the membrane or weakness of the Infant that cannot get clear thereof. And therefore herein significations are naturall and concluding upon the Infant, but not to be extended unto magical signalities.
Sir Thomas Browne, Pseudodoxia Epidemica or, Enquires into Very Many Received Tenents and Commonly Presumed Truths, London, 1658.
For an extensive treatment of the significance of the caul in early modern European folk culture, see Carlo Ginzburg, Ecstasies: Deciphering the Witches' Sabbath, University of Chicago Press, 2004 (trans. of Storia notturna. Una decifrazione del sabba, Einaudi, 1989).
(Friday Symptoms #2)