The manner in which this change in the position of the head takes place, varies a good deal in different labours: in primiparæ it usually takes place slowly, and requires several pains before it is completed; as the pain comes on, the posterior fontanelle, which was backwards and to the right, now advances more forward and comes more within reach; the anterior fontanelle, which was towards the left foramen ovale, retreats, so that when the pain has reached its maximum the head will for a moment be felt in the transverse diameter of the pelvis, and again resumes its former position as the pain goes off: with the recurrence of each pain there is a repetition of this screw-like motion, but by degrees the head not only passes from the right oblique into the transverse diameter, but from the transverse into the left oblique, so that at length the anterior fontanelle corresponds to the left sacro-iliac synchondrosis, and the posterior one to the right foramen ovale. In women who have already had children, the whole change is frequently effected during one pain, so that the head, which but a few minutes previously was presenting in what is called the third position of the German schools, will now be found to be in the second. It is to the celebrated Naegelé of Heidelberg that we are indebted for having first pointed out the uniform occurrence of this change in the second position. From his extensive and accurate observations, confirmed since by ourselves, as well as by many others, the head presents with the occiput _originally_ forwards and to the right very rarely, but passes into this position during the course of labour. No one has ever described the mechanism of parturition so minutely and correctly; and the value of his investigations is the more enhanced, when we recollect what erroneous notions have prevailed upon this important subject up to the present time. "In the former part of my practice," says this distinguished obstetrician, "not knowing that the head made this turn, I always concluded that my examinations in the early part of labour were incorrect, and was very uneasy that I did not find it all exactly as the books described, and attributed my want of success in ascertaining the position to my own awkwardness. At length in a private case, in which I was much interested, I again felt what I thought was the anterior fontanelle towards the left foramen ovale; and circumstances occurring which rendered it necessary to apply the forceps and terminate the labour, I found that the head had been actually in the position which I imagined I had felt. Since this time I have, in many cases, sat by the bed-side during the whole labour, with my finger upon the head, and thus come at the truth." (_MS. Lectures._)