The importance of this error is plain, involving as it does the narrow ing of the pelvis. (Dubois.) Its height, following the curve of the sacrum and coccyx, is about 5.8 inches.
The lateral walls are included between the lines which limit the an terior and posterior walls. They have a height of about 3.9 inches. They are divided into two inclined planes, the anterior and posterior, separated by a line which passes through the base of the sciatic spines.
The anterior plane is formed by a large part of the posterior face of the cotyloid cavity, and the posterior half of the internal face of the ischiatic tuberosity. In front it blends with the obturator fosse, which may be considered its natural prolongation.
The posterior plane is formed by the internal face of the sciatic spine, the anterior face of the two sciatic ligaments, the ischiatic foramina, the muscles and the nerve fibres which traverse these openings.
Three diameters can be distingtiished in the cavity: 1. An antero-posterior diameter, A P, which runs from the middle of the posterior face of the pubic symphysis, to the union of the second and third pieces of the sacrum. It is about 4.5 inches in length.
2. The transverse diameter, T, which runs from one side to the other, crossing the diameter A P at right angles. It is about 4.5 inches long.
3. Two oblique diameters, 0, which pass from the posterior face of each of the sub-pubic canals to the centres of the great sciatic foramina. They are about 4.5 inches.
The cavity is therefore about 4.5 inches in every direction. (Dubois and Pajot.) The superior strait (Fig. 11) is bounded' posteriorly by the sacro vertebral angle and the anterior edge of the base of the sacrum; laterally by the lines innominata; in front by the ilio-pectinal eminence, the pos terior edge of the horizontal ramus of the pubes, and the superior part of the body of the pubes, and of the pubic symphysis.
The superior strait has four principal diameters: 1. The antero-posterior diameter, A P D, sacro-pubic, sacro-sub-pubic, conjugate, conjugate vera of the Germans, the small diameter.
It extends from the superior edge of the pubic symphysis to the middle of the mere-vertebral angle. It is from 4.2 to 4.5 inches long.
Pinard holds with Schroeder, that, from the obstetrical point of view; there is an express indication for measuring this diameter not from the superior edge of the symphysis, but from the promontory to that point of the symphysis which is nearest to it. For this point is, according to Schroeder and Pinard, inch below the superior edge of the symphysis. It is this diameter which Pinard calls the minimum or useful diameter. Crouzat, continuing Pinard's measurements, admits that the post-pubic point is situated below the sub-pubic point about 4 tenths of an inch.
2. Transverse or his-iliac diameter, the great diameter, extends from the middle of the lines innominata of one side to the same point on the opposite side. It is from 5 to 5.2 inches long.
3. The oblique diameters, right and left, extend from the anterior region of one of the iliac synchondroses to the ilio-pectinal eminence of the opposite side. They are from 4.6 to 4.9 inches long.
4. To these principal diameters we may add: a. A sacro-pectineal diameter (of Burns and Pajot) running from the middle of the promontory to the superior edge of the horizontal ramus of the pubes, immediately below the sub-pubic angle. It is about 3.9 inches long.
b. A sacro-cotyloid diameter (Velpeau and Pajot), which runs from the middle of the promontory to the superior and posterior parts of the cotyloid cavity. It is about 3.5 inches long.
The circumference of the superior strait measures about 16 inches.
The inferior strait (Fig. 12) is formed by two triangles, united by a common base. This base is formed by the line which joins the two ischi atic tuberosities. The anterior triangle has its apex at the pubic arch, the posterior at the tip of the coccyx.
The diameters of the inferior strait are: 1st. The antero-posterior, or coccy-pubio diameter, which extends from the tip of the coccyx to the top of the pubic arch.
2d. The transverse or bis-ischiatic diameter. This joins the centre of 'the two ischiatic tuberosities.