62 OBSTETRICS. 



plane into the hollow of the sacrum. At the inferior strait there is 

 no difference between the two positions ; but after the escape of the 

 head, the vertex turns towards the right thigh^ the left shoulder- 

 comes under the arch of the pubis, and the right falls into the hollow 

 of the sacrum ; but in these changes there is no variation from the 

 proportional relations between the foetal head and the maternal 

 pelvis. 



This is considered by some obstetricians as not so favourable a 

 position as the first, in consequence of the impediment offered by the 

 rectum to the rotation ; this is not believed to be a valid objection, as 

 the rectum can be easily emptied, and then offers but a slight 

 obstacle. 



The third position is confessedly so rare that some obstetrical 

 writers have banished it entirely. Baudelocque admitted it rather to 

 fill up his plan, than from any evidence of his senses. Madame 

 Boivin met with it only six times in twenty thousand five hundred 

 and seventeen cases. Velpeau also is sceptical as to its existence. 



When it does occur, its mechanism is a little different from the 

 preceding ; the occiput is at the pubis, and the forehead at the pro- 

 montory of the sacrum. In this case, the occipito-mental diameter 

 corresponds with the axis of the superior strait, and the occipito- 

 bregmatic circumference with the plane of that strait. 



The bi-parietal diameter, however, is situated transversely, and 

 the occipito-bregmatic from front to rear. There is no rotation ne- 

 cessary in this case, and none occurs, consequently there is no re- 

 stitution. 



The shoulders commonly enter transversely, although they gene- 

 rally emerge with one at the sacrum, and the other at the pubis, 

 without our being able to tell beforehand which it will be. In the 

 rest of the labour there is nothing peculiar. 



The fourth position of the vertex is the most common of the occi- 

 pito-posterior varieties. In this position the same diameters corre- 

 spond as in the first ; but, the situation of their extremities is re- 

 versed. Engagement takes place more easily, and the head descends 

 into the excavation more readily than in the occipito-anterior posi- 

 tions, until it reaches the floor of the pelvis. Instead of extension 

 occurring readily now, a greater degree of flexion is demanded so as 

 to enable the occiput to rotate into the hollow of the sacrum, which 

 it does, from right to left, upon the right posterior inclined plane, 

 whilst the forehead, or anterior fontanel le, slides forward upon the 

 left anterior inclined plane, from left to right. 



The vertex is born first, over the perineum, and extension takes 

 place backwards, so that the posterior fontanelle, the sagittal suture, 

 the anterior fontanelle, the parietal protuberances, and the several 

 parts of the face are successively seen in front of the perineum. 

 Immediately after the birth of the chin, restitution takes place ; the 



