PRESENTATION OF THE VERTEX. 295 



slides from right to left and from before backwards on the right pos- 

 terior inclined plane, so as to get into the hollow of the sacrum. This 

 is the moment when the sacral plexuses are most forcibly compressed, 

 and when most women are affected with cramps in the legs and 

 thighs. 



728. Extensioyi. Hitherto the flexion of the head has gone on 

 increasing; henceforth it diminishes: the extension movement is 

 about to begin; as it approaches the inferior strait the occiput rises, 

 and by degrees causes the chin to abandon the front of the thorax; 

 instead of continuing to bend forwards, the head lends to turn 

 over backwards, in order that the occipito-mental diameter may 

 be brought into parallelism with the axis of the inferior strait, 

 without hindering the rest of the trunk from still following the course 

 of the central line of the superior strait; the rectum and neck of 

 the bladder being now more forcibly compressed than before, give 

 rise to a straining, and tenesmus; the womb and abdominal muscles 

 contract more violently than ever; the perineum distends, elongates, 

 grows thin, and in this way prolongs the posterior wall of the pelvis 

 to an extent of from three to four inches. The head being once 

 engaged in the inferior strait, is no longer in the same relation to 

 the diameters as before. Yet, it is well to observe that its great 

 diameter and occipito-bregmatic circumference have not undergone 

 any change of this sort, and that below as well as above they still 

 represent the plane and axis of the strait. But instead of being 

 &ituated obliquely, the bi-parietal and occipito-bregmatic axes are 

 so situated, that the former corresponds to the direction of the bi- 

 sciatic diameter, and the latter to that of the coccy-pubal diameter; 

 which is the reason why at the end as well as at the beginning of 

 labor the great diameters of the occipito-bregmatic circumference are 

 always found to correspond to the largest diameters of the pelvis, and 

 why the chief end of the rotation motion is to establish these favor- 

 able relations. 



729. As the rotation takes place only at the expense of a twist 

 of the child's neck, and not of its whole body, the shoulders retain 

 their primitive direction at the superior strait, so that their great 

 diameter is parallel to that oblique diameter which proceeds from 

 left to right, and from behind forwards. 



730. The efforts of the woman are now redoubled, the head 

 gradually engages in the vulva, its back part being forwards, and 

 slides down the plane presented by the anterior surface of the coccyx 

 and extended perineum, which plane is strongly inclined forwards; 

 the great labia are slowly effaced, and grow thin from their perineal 

 commissure towards their pubal extremity; the nyraphae are forci- 



