This author considers that a proportionate increase in the transverse diameter does not fully compensate for a great contraction in the conjugate, in permitting the passage of the fcetal head. In cases in which the brim is ob long, and presents a preponderance of the antero-posterior diameter, as in the funnel shaped and masculine pelves, the fcetal head necessarily presents its long antero-posterior diameter to that of the brim, and the case rnay be one of face presentation.
Distortions affecting the cavity only or prin cipally, and causing obstruction there to the passage of the fcetal head.—These are occa sionally seen witbout particular deformity at the brim or outlet.
Vertical flatness of the sacrum, or want of the proper vertical curvature, is occasionally met with, according to Dr. Churchill, after whom the annexed engraving of a specimen of this deformity is taken (fig. 115.). This is the brim measures 21 inches ; the transverse 5 inches ; and the distance from the side of the sacral promontory to the centre of the su perior branch of the pubis on each side is equal, and measures also 21 inches. Dr. Ramsbotham considers this to be just below the smallest space through which the fcetal head could pass entire. The cavity, however, and the sacral curve, are well proportioned, and the distance between the sciatic tuberosi ties 41 inches, or a quarter of an inch more than in a healthy pelvis ; so that the fcetal head would pass easily, having overcome the ob struction at the brim. The second is the pelvis of a woman who was delivered by cranio tomy, which is considerably contracted in all sometimes accompanied by an increased sacro vertebral angle. One or two instances I have met with, in which there was a tendency to this peculiarity. It is attended with diminu tion of the antero-posterior diameter of the cavity, and is most frequently met with in cases of general ovate deformity.
The opposition it would offer to the passage and circumvolution of the fcetal head is evi dent.
An inward projection of the sciatic spines is often seen in connection with contraction of the transverse diameter of the inferior outlet, without any inordinate massiveness or mascu line form of the pelvis, as in the example be fore mentioned, and is, probably, sometimes dependent on the same cause—viz., the action of the great glutei muscles upon softened ischia. In some instances, however, this length and projection of the spines cannot be thus accounted for. Obstruction may occur before the head reaches the tuberosities in these cases.
Their relative positions may be altered also by fracture.
Pelves in which the cavities are,contracted at the lower part by disease, present the chief obstacles at the inferior outlet, and will be considered under that head.
A variety of the shape of the pelvic cavity is mentioned by Murphy, as forming a contrast to the funnel-shaped masculine pelvis. It is the funnel-shaped reversed by the gradual wi dening of the transverse diameters downward ; but, being generally attended by some contrac tion at the brim, it belongs rather to the classes of deformities before described, and would seem to resemble the first case of Ramsbo tham's there cited, and to be the beginning of a more complete ovate deformity, afterwards to be mentioned, as shown by the widening of the inferior opening.
Distortions affecting the outlet only or prin cipally.— In these cases the greatest obstruc tion occurs at the inferior outlet, which, being comparatively independent of the brim and calcity, may. be contracted without any im portant alteration in their shape or size.
Contraction of the transverse diameter is tbe most frequently seen. The ischial tube rosities are approximated, and the space of the sub-pubic arch lessened ; and thus, indi rectly, the antero-posterior diameter is ren dered less effectual. Cases are frequently presented to the obstetrician, and many are on record, in which obstruction has occurred at the sciatic tuberosities, and the use of the forceps been rendered necessary. The normal distance between the centres of these processes is from 4 inches to 41. In my measurements I have met with as small a distance as 3,1 inches in female pelves, in which the trans verse diameters were generally rather small.
The bi-parietal diameter of the fcetal head is said to be 3,-} inches, and is placed obliquely between these tuberosities, as the occiput emerg,es under the sub-pubic arch. The soft parts, besides, will occupy at least thi ee fourths of an inch. Thus with a large and well-ossified head and contracted inter tuberal distance, the impediment is great. The diminished span of the sub-pubic arch, also, pushes back the head upon the coccyx, and renders a greater enlargement than usual of the antero-posterior diameter necessary ; while at the saine time the great sacro-sciatic ligaments are also approximated and rendered extremely tense, and thus the oblique di ameter between them and the ischio-pubic ransi is also considerably diminished.