PELVIC PRESENTATIONS. 



77 



21. 



To simplify the matter, however, most of the recent writers reduce 

 them to two, viz. : sacro- 

 anterior, and sacro-pos- Fig. 

 terior, that is, the back 

 of the child towards the 

 belly of the mother, and 

 the back of the child 

 towards the back of the 

 mother. Not that the 

 back of the child is di- 

 rectly anterior or poste- 

 rior, but oblique, the 

 transverse diameter of 

 the child's hips corre- 

 sponding to one or other 

 of the oblique diameters 

 of the superior strait. 

 (Fig. 21.) 



Meclianism. — The 

 mechanism of the sacro- 

 anterior positions is so 

 much alike, that but one 

 description will be given 

 of them. Naegele, on the 



mechanism of parturition, says, that "In every case, whether the 

 nates have at first a completely transverse or oblique direction, they 

 will be always found, on pressing lower into the superior aperture of 

 the pelvis, to have taken an oblique position ; and that ischium which 

 is directed anteriorly, to stand lowest. In the most common position, 

 where the sacrum of the child is towards the left acetabulum, as the 

 breech descends into the pelvis, it is the left ischium that stands 

 lowest, and is first touched by the finger. The child's left hip rotates 

 forwards upon the right anterior inclined plane under the arch of the 

 pubis, while the right hip slides backwards into the hollow of the 

 sacrum along the left posterior inclined plane. As the shoulders are 

 supposed to remain fixed in the uterus, flexion of the child's body 

 takes place as it is being born, and restitution after the birth of the 

 hips is effected. As the shoulders also engage obliquely, they un- 

 dergo rotation, the left shoulder rotating on the right anterior inclined 

 plane, and appearing under the pubis, while the right falls into the 

 hollow of the sacrum (Fig. 22). 



*' But whilst the shoulders are descending in the above-mentioned 

 oblique position, the head, with the chin resting upon the breast, 

 presses into the superior strait in the direction of the right oblique 

 diameter, that is, with the forehead at the right sacro-iliac junction, 

 the occiput rotates upon the left anterior inclined plane, towards the 



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