A textbook of obstetrics . in anterior positions,—namely, whatever portion ot thefetal body first strikes the resistance of the pelvic floor, whether itencounters this structure behind or in front of the median transverse ABNORMALITIES IN MECHANISM. 3T line, is directed forward, inward, and downward, under tlie arch of the symphysis. As the occiput Is the most dependentpart of a vertex presentation, it must first encounter the resistanceof the pelvic floor, and must, accordingly, be rotated in the direc-tions named. Abnormalities in Mechanism.—Backward rotation of the occi-put complicates labo
A textbook of obstetrics . in anterior positions,—namely, whatever portion ot thefetal body first strikes the resistance of the pelvic floor, whether itencounters this structure behind or in front of the median transverse ABNORMALITIES IN MECHANISM. 3T line, is directed forward, inward, and downward, under tlie arch of the symphysis. As the occiput Is the most dependentpart of a vertex presentation, it must first encounter the resistanceof the pelvic floor, and must, accordingly, be rotated in the direc-tions named. Abnormalities in Mechanism.—Backward rotation of the occi-put complicates labor by protracting its course, increasing thedanger of fetal death, and subjecting the mother to increased riskof injury. The causes may be divided under three heads : Anomalies of Force.—Anterior rotation is the resultant of the forces of expulsion and resistance ; hence, any condition disturbing the normal relation of these forces interferes with the normal rotation. Thus, backward rotation occurs if there is dimin-. Fig. 218.—Posterior positions of a vertex presentation. ished expulsive power, increased resistance or decrease in re-sistance, as occurs in cases of very large pelves, relaxed pelvicfloors, small and yielding heads. Anomalies of Flexion.—If flexion is imperfect, the anteriorvault of the cranium (as in those rare cases of presentation ofthe large fontanel), the brow, or the chin first strikes the pelvicfloor, and is, therefore, directed forward, and the occiput is thusdirected backward. Insuperable Obstacles to Forward Rotation.—In some cases itflexion is only fairly good, and the occiput dors first strike the pel-vic floor, the occiput rotates backward, because the large diam-eter of the head (fronto-occipital, 1 1 34 em.—453 in.) is engaged,and rotation from one oblique diameter of the pelvis to the other i68 THE MECHANISM OF LABOR. oblique is impossible, on account of the very tight fit (A thehead in the pelvis. The occiput is also directed backwar
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Keywords: ., bookcentury1800, bookdecade1890, bookidtex, booksubjectobstetrics
