Dissection of the torso of a pregnant woman, showing the internal organs and the foetus. Colour engraving by J. Pass after D. Dodd, 1794.

Sibly describes the action of quickening as an event that occurs in the fifth month of pregnancy. It is an "instantaneous, yet undescribable motion of the vital principle, which, the instant the foetus has acquired a sufficient degree of animal heat, and is completely formed in all its parts, rushes like an electric shock, or flash of lightning, conducted by the fanguiferous and nervous fluids, from the heart and brain of the mother, to the heart and brain of the child. At this moment the entire circulation begins; the infant fabric is com pletely set in motion; and the child becomes a living soul. As soon, therefore, as the circulation commences, the child starts into life; and the instant the circulation ceases, life ceases also. This act of quickening is therefore derived from the blood, and is so sensibly felt by the mother, that she often faints, and feels an internal depression of her animal and vital powers, which may be said, in some measure, to have departed from her. ... I have, to the best of my ability, endeavoured to illustrate this occult process of nature, by means of the annexed copper-plate engraving, taken from a drawing of the viscera and womb of an unfortunate female, who fainted and died at the time of quickening; the foetus itself being now preserved in spirits."--Sibly, op. cit., pp. 116-118