. The science and art of midwifery . the long endof a lever, and works, therefore, at agreater mechanical advantage thanthat directed against the occiput,*At the same time, if the anterior wallbe divided by a line drawn on a levelwith the lower margin of the sym-physis, we find that in the superiordivision the general pelvic pressure di-minishes from before backward, whilebelow the line indicated, owing tothe open space afforded by tin1 pubicarch in front, pressure diminishes from behind forward. Now, inaccordance with the mechanical principle that when a body is sub-jected to varying pressure
. The science and art of midwifery . the long endof a lever, and works, therefore, at agreater mechanical advantage thanthat directed against the occiput,*At the same time, if the anterior wallbe divided by a line drawn on a levelwith the lower margin of the sym-physis, we find that in the superiordivision the general pelvic pressure di-minishes from before backward, whilebelow the line indicated, owing tothe open space afforded by tin1 pubicarch in front, pressure diminishes from behind forward. Now, inaccordance with the mechanical principle that when a body is sub-jected to varying pressures the movement will take place in the direc-tion of the least pressure,! we find that the frontal portion, which liesabove the sub-pubic plane, turns backward, while the occiput, whichlies below, turns under the arch of the pubis. It must m»t he supposed, in .imagining the results of rotation, thatthe movement continues until exact coincidence of the sagittal sutureand the conjugate is reached. Irishman endeavored t«» measure the. Fig. 101—O, B, short cud of the headlever; B, F, long end of head lever.(Tarnier et Chantreuil.) * Tarnteb ft Chantbeuil, Train <!<• Iarl dea accouchements, p. <» 1L+ Stephenson, On the Mechanism of Labor, Obstet. Jour, of Gr. BrOctober, 1878, p. W5. and Ire, MECHANISM OF LABOR. 177 divergence between the two after the head had escaped from under thepelvic arch, by stretching a cord over the surface of the head from thelower border of the symphysis to the coccyx. He found that in leftoccipital positions the cord crossed the lambdoidal suture aboui .in inchto the right of the small fontanelle, and thence extended forward tothe middle of the opposite orbit, intersecting the median line at ornear the anterior fontanelle.* In emerging from the pelvis, the two tubera parietalia do not passout at the same time. In place of this, the head rolls upon its side, sothat in left occipital positions the presentation is formed by the upperand poste
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