60 OBSTETRICS. 



monly presents itself in the occipito-anterior, rather than in the con- 

 trary way. The child in utero naturally lies on its back, with the 

 head inclined downwards towards the orifice of the womb. The 

 long diameter of the womb, at the end of pregnancy, and especially 

 at the commencement of labour, dips at an angle of about forty-five 

 degrees towards the horizon in a woman who is sitting or standing 

 up. The child being in a complete state of flexion both as to its 

 body and limbs, would naturally roll, so as to bring that part of its 

 body which is convex, to adapt itself to the concavity of the womb. 

 This will of course be its back. The uterus at the same time rests 

 upon the abdominal muscles anteriorly, which form a soft cushion 

 for it. The head can only pass the brim of the pelvis, (superior 

 strait,) by the adaptation of certain of its diameters to those of the 

 pelvis. The longitudinal diameter of the child's head is four and a 

 half inches, whilst the antero-posterior diameter of the superior 

 strait is only four; it must therefore adapt itself either to the oblique 

 or the transverse ; of these, the oblique is most common. Suppose 

 it to be a first position, in which the longitudinal diameter corre- 

 sponds with the oblique of the superior strait, the vertex is behind the 

 left acetabulum, and the forehead at the right sacro-iliac junction. 

 The two fontanel les will be at first on a level. 



The first step in the mechanism of labour is i\\e flexion of the head, 

 or the approach of the chin to the breast. This is produced by the 

 action of the uterus pressing the head downwards into the cavity. 

 Being pressed from above downwards, the spine causes the head to 

 bend forwards, so that the occiput sinks towards the centre of the 

 pelvis, and the chin is squeezed firmly against the breast. 



By this movement, the diameters of the child's head are brought 

 into correspondence with such diameters of the pelvis as will allow 

 its ready transmission. The occiput occupies the centre of the 

 superior strait ; the occipito-bregmatic diameter corresponds to the 

 oblique diameter of the pelvis, which extends from left to right, and 

 from before backwards; the bi-parietal, represents the other oblique 

 diameter ; the occipito-mental is parallel to the axis of the pelvic 

 circle, and the occipito-bregmatic circumference corresponds to the 

 plane of the strait. 



Next follows rotation, by which is meant, the screw-like move- 

 ment of the head in the excavation by which the vertex is brought 

 to the symphysis pubis. 



By the continued action of the uterus, the head is pushed into the 

 excavation until it reaches the floor of that cavity ; here its progress 

 is arrested and the direction changed by the gliding of the occiput 

 upon the left anterior inclined plane, from behind forwards and from 

 left to right, so as to place itself behind the symphysis pubis. Whilst 

 the vertex is executing this movement, the forehead glides from right 

 to left, and from before backwards, on the right posterior inclined 



