_Relative frequency of the different positions._--The most favorable positions, like the most favorable presentations, are also the most frequent. According to Baudelocque, in _ten thousand three hundred and twenty-two cases_, of head presentation, there were _eight thousand five hundred and twenty-two_ cases when the back of the child's head was on the mother's left side, and towards the front, (or in the left anterior occipito iliac position); _one thousand seven hundred and fifty-four_ when it was on the _right_ side towards the front, (right anterior occipito iliac); _twenty-five times_ to the right side, but towards the Sacrum, (right posterior occipito iliac); and _nineteen times_ on the left, but towards the Sacrum, (left posterior occipito iliac.) Being most frequently with the back of the head _towards the front on the left side_, as shown in Plate XXVII; next towards the front on the right side; and but seldom towards the Sacrum, or back, on either side. In all these ten thousand cases we do not find a single instance of the head lying from back to front, in the _occipito pubic_, or _occipito sacral_ positions, commonly called transverse; neither do we find a single instance in _fifteen thousand six hundred and fifty-two cases_ recorded by Madame Lachapelle; which will show how rare such unfortunate positions must be. What this great frequency of one particular position depends upon we do not know--possibly on that cause, previously alluded to, which determines the most frequent presentation. In the next Chapter, the mechanism of delivery, or the manner in which the child escapes out of the body, as it most frequently occurs, will be fully explained. CHAPTER XI. THE MECHANISM OF DELIVERY, IN A PRESENTATION OF THE HEAD. THE LEFT ANTERIOR OCCIPITO ILIAC POSITION. This is the presentation and position most frequently observed, perhaps fifteen out of sixteen times. In most Obstetrical works it is called the _first_ position. By observing the following Plates, and referring to the previous explanations of the diameters of the Pelvis, and fœtal head, in Plates IX and XXV, it will be seen that through all its changes of position, while making its exit, the head always presents by one of its shortest diameters to one of the largest diameters of the Pelvis, so that the relation between them is invariably the best that could be established; and many peculiar turnings and revolutions occur, apparently for the express purpose of bringing this about.