There can be no doubt that, among the numerous matters at present occupying the attention of obstetricians, none is more important than the subject of this paper. So evident is the correctness of this statement that one cannot but wonder why attempts to arrive at the truth have been, so far as we know, delayed till the present day. It is long since excellent researches of an analogous kind in regard to the force of the circulation of the blood, the power of the ventricles of the heart, were published; yet such researches do not seem naturally so attractive, nor do they give promise of so valuable practical results as those into the power of labour.