Each of these presentations has two corresponding positions, determined by the side on which the child's head lies. If the head be on the mother's right side it is called the _right cephalo iliac position_, and if it be on the left side it is denominated the _left cephalo iliac position_. The mechanism of spontaneous delivery is the same in them both, and in all their varieties, and so is the mode of rendering assistance, so that a description of one will suffice. Sometimes, when the labor has lasted long without assistance, one arm will be forced down first, and even appear externally. This used to be considered a separate presentation, and described as such, under the name of _presentation of the hand and arm_. There is no reason for describing it separately however, and no utility in doing so, as it differs in no essential particular from ordinary presentation of the shoulders, and must receive the same assistance. What it is that produces presentations of the Trunk, and other unfavorable parts, is not known, though they are generally thought to be owing to excessive motion in the child, or obliquities of the Womb. M. Lachapelle met with _sixty-eight_ cases of Trunk presentation in _fifteen thousand six hundred and fifty-two labors_, which is nearly the same as the face. The right side presents more frequently than the left, and the head is on the left side oftener than on the right, as it is in ordinary head presentations. As a general rule assistance is always rendered in presentations of the Trunk, and is generally considered absolutely necessary. It is undoubtedly true however, that nature has effected delivery in such cases unaided, though rarely, and such instances are considered as extremely fortunate exceptions to the general rule. M. Chailly says that the accoucheur should never leave such cases to nature alone, but always aid her; but other authors trust to her a little more. The most usual mode of rendering assistance is to _turn the child_, and bring down the feet, a manœuvre which will be fully described hereafter. In some cases the child turns itself, from the contractions of the Womb, before it enters the upper strait; and in other cases, when very small, or long dead, it will pass folded double. This self-turning however, cannot take place after the escape of the waters, so that it seldom occurs when the membranes are broken. MECHANISM OF DELIVERY IN PRESENTATIONS OF THE TRUNK BY SPONTANEOUS EVOLUTION. This is the most usual mode for the fœtus to escape, in each presentation, and in every position. By referring to the following Plates, and the accompanying descriptions, it will be readily understood. [Illustration: PLATE XXXV. _Position of the Fœtus in a presentation of the right shoulder, and in the left cephalo iliac position._]