Previous to the rupture of the membranes the child's body lies _across_, as formerly explained; but immediately after the rupture the shoulders descend into the Pelvis, as seen in the above Plate, while the head remains above the pubes; the arm frequently, but not always, protruding externally. The shoulder then continues to descend, the body following, bent up against the face, as seen in Plate XXXVI. [Illustration: PLATE XXXVI. _Descent of the shoulder in a Trunk presentation, at a more advanced period._] Here the shoulder is protruded from the Vulva, the back being nearly folded, and the knees turned up against the face. [Illustration: PLATE XXXVII. _Descent of the shoulder and trunk at a still later period._] The whole Trunk is now fully delivered, folded almost double, and the legs and feet are turned up against the face. They speedily follow however, and then nothing is left but the head, and perhaps one or both arms, placed against the sides of it, as shown in Plate XXXVIII. [Illustration: PLATE XXXVIII. _The Trunk has fully descended, and only the head is left, with one arm._] The arm is generally very easily brought down, or it may remain and come with the head. The delivery of the head is effected the same as in presentations of the pelvis, and is seldom attended with much difficulty, the parts having been so much distended. The body always rotates so that the back comes in front, and the chin passes into the curve of the Sacrum. This is the way in which the delivery is effected by nature in such cases, and it will readily be conceived how dangerous it is to both mother and child, and how seldom it can be accomplished. If the Fœtus be of a full size, and the mother's pelvis no larger than ordinary, it is almost impossible for this spontaneous evolution to take place; and even when it does, it is with the greatest difficulty, the mother suffering in an extreme degree, and running great risk, not only of the most serious after results, but even of death. To the child the danger is equally great, owing to the severe and long-continued compression it receives, and the unnatural position it assumes. M. Velpeau tells us that in _one hundred and thirty-seven_ such cases, _one hundred and twenty-five_ of the children died. The number of the mothers also, who either died or were made sufferers all their future lives, was undoubtedly great, though unknown. It is evident therefore, that presentations of the Trunk are the most unfavorable known, and labor in them is but rarely spontaneous. Nature can but very seldom effect the delivery of the Fœtus herself, and even when she does it is with the greatest risk, both to it and the mother. The accoucheur should _always_ assist therefore, if he can, because even if nature can complete the delivery it is with such danger. The means of assisting, by turning, will be described in another Chapter.