PRESENTATION OF THE VERTEX. 303 



varieties of this position differ only by slight shades from each other, 

 I nevertheless think, but merely for the purpose of not deviating too 

 far from generally adopted opinions, that I ara bound to give a suc- 

 cinct explanation of its peculiar mechanism. 



A. First Variety. 



Left fronto-acetabular position. 



4th position of Baudelocque, Gardien, Dubois, Desormeaux, Lebreton, Fla- 



mant, Madame Boivin: 3d of Maygrier, Capuron, Dug6s, and Madame Lacha- 



pelle: 109 in 20,517 cases, Madame Boivin; 164 in 22,343 cases, Madame La- 



chapelle. 



743. The left fronto-acetabular position is the most common of 

 the three posterior varieties. It unites all the most favorable condi- 

 tions of its species, and in this respect it excels all the others. The 

 back of the foetus being turned backwards and to the right, the ab- 

 domen towards the front and left, its left side to the front and right, 

 and its right side behind and towards the left side of the womb, 

 engages in the superior strait in such a way that the occipito-mental, 

 bi-parietal, and occipito-bregmatic diameters, and occipito-bregmatic 

 circumference are parallel with the oblique diameters, the plane, and 

 axis of that strait, respectively, as in the first anterior position. 

 There is this difference, however, that the frontal extremity of the 

 occipito-bregmatic axis occupies the place of the sub-occipital ex- 

 tremity, that the left extremity of the bi-parietal diameter has assumed 

 that of its right extremity, that the anterior fontanel glides behind 

 the ilio-pectineal eminence, instead of descending before the sacro- 

 iliac symphysis, and that the posterior fontanel, instead of being 

 slightly inclined in front and towards the left, is, on the contrary, 

 turned more or less backwards and towards the right, which, as is 

 evident, does not at all interfere with the proportional relations of the 

 head and pelvis, and prove, that so far the posterior are not more 

 unfavorable than the anterior positions. 



744. After the dilatation of the neck and the rupture of the mem- 

 branes, when the occipito-bregmatic circumference has passed the 

 superior strait, the head, meeting with a deep excavation behind, is 

 rapidly urged to the very bottom of the excavation, and at first the 

 labor seems to progress more rapidly than in the very opposite posi- 

 tion; but, from this moment, the difficulties mentioned above become 

 more and more manifest. Instead of being gradually replaced by 

 the act of extension, as in the anterior positions, the flexion still 

 continues to increase with every pain; whilst the forehead is arrested 

 behind the pubis and the occiput is abutted against the front of the 



