_Second Movement, Rotation of the Head._--When the Fœtus is thus brought to the bottom of the Pelvis, its head turns completely round, the back of it being brought to the front, or under the pubes of the mother, and its forehead turned against her Sacrum, as represented in Plate XXIX. [Illustration: Plate XXVIII. This Plate represents the head descended still lower in the Pelvis.] PLATE XXIX. Here the head is seen just beginning to turn--the right side, and part of the back of it, just passing under the pubes. As the rotation becomes complete the neck straightens, so that the two fontanelles are again found on the same level. Finally the back of the head fully emerges from under the pubic arch, and the chin slides gradually out after it beneath, so that the neck of the child is encircled by the ring of the Vulva. [Illustration: Plate XXIX. This Plate represents the head still further down, and beginning to Rotate.] [Illustration: PLATE XXX. In _Fig. 1_, the back of the head is nearly under the pubic bone, while the forehead is just passing the external opening below. In _Fig. 2_, the back of the head is completely extruded, and also the chin, so that the whole head is now born.--(_p._ is the pubic bone, in front.)] In Fig. 1 it will be seen how the Os Coccygis, or lowest part of the back bone, (c,) is straitened out, backward, while the head is passing, as I explained before; and in Fig. 2 it has returned again to its natural position. The reason for this rotation of the head will be obvious on calling to mind the form of the Pelvis and the external opening. On examining Plates IX and X, it will be seen that the longest diameter of the Vulva, or external opening, the _antero posterior_, is nearly at right angles to the longest diameters of the upper strait, the _oblique_. Now the longest diameter of the head is adapted to this oblique diameter, on _entering_ the upper strait, as already explained, and it is necessary for it also to be adapted to the longest diameter of the external opening, the anterior posterior when making its _exit_; but as the two are not parallel the head is compelled to turn, or rotate, in order to pass from one to the other. To accomplish this however, the neck has to be twisted considerably. But when the head is fully delivered, it is immediately straitened again, by the back of the head turning towards the mother's left side, so that its proper relations with the trunk is re-established.