The muscles:before enumerated, which sup port the erect posture, as they are in the adult rnore powerful and developed, have a corre sponding effect in increasing the contraction of the diameters consequent on the distortion. The bones yield between their contracting dis tances in the direction already impressed upon them. The acetabula are pressed backward by the psom and iliacus muscles, and the ischial tuberosities and trochanters approxi mated by the pressure of the great glutei, which, aided by the pyrifonnes, will also draw for ward the lower part of the sacrum and coccyx. The powerful influence of the adult muscles upon the pelvic bones partially softened, and especially that of the great glutei upon those bounding the diameters of the inferior outlet, will produce many of the pa tial deformities before treated of, as the influence of 'mechan ical posture in a limited extent, or short dura tion of the softening disease, will produce others, principally those of the pelvic brim.
The peculiar variety of the partial defonnity will depend upon the frequency of the use of one particular posture or set of muscles ; and this will depend chiefly, in the child, on the concurrent ailments which usually affect it, and in the adult on the nature of his or her habits and employment.
The degree of the backward curvature of the cotylo-sacral arch seems to depend upon the degree of anterior lurnbar curvature, which necessitates a forward projection of the femurs to keep the line of gravity between the feet (see ,fig. 122. E, a b).
The rostrated pelvis, with elongated antero posterior diameters, apparently results from the coincidence of the softened pubes with the causes of oblong deformity before adverted to, as produced by a backward spinal curve, causing the line of gravity to fall considerably behind the acetabula, and dragging backwards the superior part of the sacrum.
The mechanism of these important pelvic deformities has been entered into more in detail because of the evident practical infer ences which may be drawn from it with re gard to the treatment and position of children, especially females, afflicted with rickety dis ease.
Degree of obstruction.— Pelves affected by the foregoing distortions are usually arranged by British obstetricians, according to the de gree of obstruction at the brim, into three classes :— 1st. Those which will suffer the full-sized fcetal head to pass entire.
2nd. Those through which delivery may be accomplished "per vias naturales," by means of premature labour, craniotomy, or mutilation of the foetus.
3rd. Those in which the degree of defor mity is so extensive as to call for the Cmsarian section, or the very early induction of abor tion.
The limits of the first class have been va riously stated by different obstetricians, accord ing to their opinions regarding the obviou variations in size of the fcetal head, and its de gree of ossification. The following list conveys the opinions of the most eminent authorities upon the lowest limits through which the fcetal head can pass entire :— In these cases, according to Ramsbotham, it is rare that the transverse diameter does.not exceed three inches. Less room is required if the brim alone be distorted, according to the same author.
All pelves contracted in their diameters be low the measurements given in the last list may undoubtedly be considered to require, for the delivery of a fcetus of viable or full-grown size, the abdominal section.
Dr. Robert Lee, however, advocates strong ly, and with great justice, the propriety of inducing abortion in these deplorable cases, as a means of saving the life of the mo ther. When the sacro-pubic diameter is below 11 inch at the brim, this author considers that abortion should be induced before the fifth month. According to Ritgen, labour should be induced in the twenty ninth week, when the sacro-pubic diameter is 2 inches 7 lines ; in the thirtieth week, when it is 2 inches 8 lines ; in the thirty first, when 2 inches 9 lines ; in the thirty fifth, when 2 inches 10 lines ; in the thirty sixth, when 2 inches 11 lines; and in the thirty-seventh, when exactly 3 inches. When above 3 inches, the case should be left to na ture. Barlow thinks that premature labour should be induced when this diameter is con tracted to 2f or 2/ inches-4s But in many cases, especially on the Con tinent, a much less degree of contraction of the conjugate diameter has been thought suf ficient to justify the Cmsarian operation. In a table given by Velpeant, out of sixty-two cases where narrovimess of the conjugate diameter was the reason adduced for adopt ing this operation, in one case it was I inch only ; in eight cases, It inch ; in twenty three cases, I I to 2 inches ; in twenty-five cases, 2 to 2t inches ; and in five cases, 21 to 21 inches. These, without doubt, in clude many which the British practitioner would place in the first of the foregoing classes, and were adopted with a view of saving the child's life, at an additional risk to the mother.