. An American text-book of obstetrics. For practitioners and students. wide space afforded by the anterior portionof the pelvis (Fig. 258). Two factors of difficulty are thus produced : first,the widest portion of the fetal head finds itself in apposition with a narrowportion of the pelvis, and therefore requires a powerful driving impulse toforce it through the brim ; second, this retarded widest portion of the headis situated on the occipital end of the head lever, while the sincipital end isalmost free. This situation, therefore, always tends toward a too rapid descentof the sinciput—that i
. An American text-book of obstetrics. For practitioners and students. wide space afforded by the anterior portionof the pelvis (Fig. 258). Two factors of difficulty are thus produced : first,the widest portion of the fetal head finds itself in apposition with a narrowportion of the pelvis, and therefore requires a powerful driving impulse toforce it through the brim ; second, this retarded widest portion of the headis situated on the occipital end of the head lever, while the sincipital end isalmost free. This situation, therefore, always tends toward a too rapid descentof the sinciput—that is, toward the production of extension—but the degree 444 AMERICAN TEXT-BOOK OF OBSTETRICS. of extension produced varies with the relative sizes of the pelvis and thehead. If the disproportion between the biparietal diameter of the head and theportion of the pelvis in which it finds itself (that is, A, Fig. 258) is notextremely great, the production of au extension sufficient to cause a lightpressure of the forehead against the pubes may be enough to equalize the. Fig. 258.—Adaptation between the fetal head and the brim of the pelvis in posterior positions of the occiput. resistances at the opposite ends of the cephalic lever, and may thus permit thegreater propulsive force applied to the occiput (see page 433) to accomplishits descent while the sinciput is still above the brim. The head in this casewill enter the excavation in a fairly well flexed condition. If the disproportion between the occiput and the posterior portion of thepelvis is more extreme, the process of extension will continue until the occipito-frontal diameter occupies the first oblique diameter of the brim. The headmay then pass the brim, after long labor, in an extended position ;* it may bearrested at the brim by becoming a brow presentation, or it may exceptionallybe converted into a face presentation. Passage of ike Excavation.—After its escape from the superior strait thehead occupies the first ob
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