. The science and art of midwifery . f. Geburtsk., Bd. \xvii, 18G0. p. 446. ABNORMALITIES OF THE FCETUS. 565 contracted pelvis is not an insurmountable obstacle to spontaneousevolution, provided the conjugate diameter be alone shortened. Noris small size of the foetus essential to the occurrence of the process inquestion. Spiegelberg* states that it is often observed in cases wherethe foetuses are mature and well developed. Softness and compati-bility of the child naturally favor the production of spontaneous evo-lution, as is demonstrated by its frequent occurrence when the productof concepti
. The science and art of midwifery . f. Geburtsk., Bd. \xvii, 18G0. p. 446. ABNORMALITIES OF THE FCETUS. 565 contracted pelvis is not an insurmountable obstacle to spontaneousevolution, provided the conjugate diameter be alone shortened. Noris small size of the foetus essential to the occurrence of the process inquestion. Spiegelberg* states that it is often observed in cases wherethe foetuses are mature and well developed. Softness and compati-bility of the child naturally favor the production of spontaneous evo-lution, as is demonstrated by its frequent occurrence when the productof conception has undergone maceration. Mechanism.—The mechanism of the former and more ordinaryvariety of spontaneous evolution is as follows: The presenting shoulderis forced into the depths of the true pelvis by the violence of the uter-ine contractions, instead of being diverted laterally, as is the case inspontaneous versions and becomes firmly fixed beneath the symphysis,while the corresponding arm protrudes through the vulva. The body. I Fig. shoulder presentation. Section through frozen corpse. (Chiara.) of the foetus is then so forcibly flexed that the breech and the head hein close proximity to each other. The former is in contact with thesacro-iliac synchondrosis, while the latter is immovably held betweenthe breech and the upper border of the symphysis. The neck andshoulder, which rest against the lower border of the symphysis, nowbecome the pivot upon which the foetus rotates. The trunk of thefoetus is driven beyond the shoulder, and the thorax, breech, and legsare horn in the order named. The other shoulder then follows, andthe head is finally expelled. The mechanism of the second variety of spontaneous evolution,designated by Roderer evolutio conduplicato corpore? which is much* Spiegelberg, Lehrbuch, p. 541. 566 THE PATHOLOGY OF LABOR. rarer than the former, differs from it in some essential features. It isgreatly facilitated by softness and compressibilit
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