GENERAL DEVELOPMENT OF THE PELVIS. — In common with the inferior extremities, the pelvis, in the infant, is more tardy in arriving at adult perfection than the upper parts of the body ; and this delay is more evi dent in the inferior or true than in the su perior pelvis, and considerably facilitates its transit through the maternal structures. At birth, the iliac wings are flat, and their sur faces are directed more forwards and less in wards than in the completely formed pelvis. From the narrowness of the sacrum, and the shortness of the pubes, the transverse di mensions of the brim and cavity are very small, and the antero-posterior diameter, from this cause, is larger than the lateral, by to an inch. The shape of the superior open ing is less rounded than in the adult, being of a sub-quadrate rather than an oval form. The cotyloid part of the ilia is completely cartilaginous, contracted, and less projectmg, while the pubic arch is narrow and angular, and the tuberosities of the ischia are near each other so as to present a small opening at the inferior outlet. Seemmerring remarks that the obturator foramen is more elliptical in the infant than in the adult. The depth and general appearance of the true pelvis is smaller than is proportionate to the iliac wings ; and it is of nearly equal breadth throughout.
The parallelism of the lateral, as well as of the anterior and posterior pelvic walls is, I think, sufficiently marked and general to be considered as a characteristic of the con formation of the infant pelvis, as we shall find it to be of that of most of the lower animals, giving to it a square-sidedness which is well seen in the adjoining figure.
The sacrum and coccyx in the child at birth are much less curved, vertically, than they afterwards become, which causes the posterior wall to be longer than is propor tionate. The coccyx, indeed, in many in stances I have seen, was almost vertical. The sacro-vertebral angle is consequently much less marked than in the adult. Doubt less, muscular action, increasing as the de velopment of the bones progresses, has a great effect in producing the diminution of the sacro-vertebral angle backwards in after life.
It is commonly stated by anatomists, that the infant pelvis is more obliqaely placed on the spinal column than the adult pelvis. The inclination of the superior plane in the child has been placed by the b-others Weber at an angle of 151'63 with the transverse vertical plane. This is somewhat /cgs than the in
clination in the male, according to the same observers, viz. 155°.
The following table is the result of the measurements of the pelvic angles of five infants, tnade to ascertain the correctness of this statement. The angles w ere carefully taken, with much precaution against any ab normal displacement, so readily occurring in the pliant structures of the infant, by making an antero-posterior vertical section of the pelvis and whole spinal column with the whole of the soft parts attached, and in such a manner as would have tended rather to in crease, than to diminish, the pelvi-vertebral angle.
It will be remarked that the greatest dif ference from the adult is observable in the sacro-vertebral angle, which is from 10° to 15° greater than the average female adult, and from 23° to 28° greater than the average male adult.
I should here state, also, that the results of my own measurements of the angle of the superior pelvic plane in adult male subjects, have given somewhat less angles than that stated by Weber.
According to Cruveilhier, a horizontal line, from the upper border of the pubis, meets the posterior wall much lower in the infant than in the adult, though the point at which he places it in the adult, viz., a little below the base of the sacrum, is much too high in the natural position of the pelvis, as will be seen by inspection of the diagram (fig. 84.). In all the infant pelves I have just given, the tip of the coccyx reached as low as the lower border of the symphysis pubis ; both these points exactly coinciding with a line drawn perpendicular to the transverse verti cal plane. This may, perhaps, be attributed to the greater flatness of the sacro-coccygeal wall in the infant, extending it further down ward. In IVo. 5. the male child at full term, the angle of inclination of the pubic symphysis to the transverse vertical plane was only 25°, but in the last female child it was, 40°, both being less than the rnean adult angle, 50°, before given, and showing, like the sacro vertebral angle, a greater tendency to parallel ism with the spine, as in the inferior animals, an analogy which is also seen in the elongated conjugate diameter.