THE PELVIS. 181 



ischia. These eminences are separated by three notches ; one in front, the pubic arch, 

 formed by the convergence of the rami of the ischia and pubic bones on each side. 

 The other notches, one on each side, are formed by the sacrum and coccyx behind, 

 the ischium in front, and the ilium above ; they are called the sacro-sciatic notches ; 

 in the natural state they are converted into foramina by the lesser and greater 

 sacro-sciatic ligaments. In the recent state, when the ligaments are in situ, the 

 outlet of the pelvis is lozenge-shaped, bounded in front by the subpubic liga- 

 ment and the rami of the os pubis and ischium ; on each side by the tuberosities 

 of the ischia ; and behind by the great sacro-sciatic ligaments and the tip of the 

 coccyx. 



The diameters of the outlet of the pelvis are two, antero-posterior and trans- 

 verse. The antero-posterior extends from the tip of the coccyx to the lower part 

 of the symphysis pubis ; its average measurement is three and a quarter inches in 

 the male and five in the female. The antero-posterior diameter varies with the 

 length of the coccyx, and is capable of increase or diminution on account of the 

 mobility of that bone. The transverse extends from the posterior part of one 

 ischiatic tuberosity to the same point on the opposite side : the average measure- 

 ment is three and a half inches in the male and four and three-quarters in the 

 female. 1 



Position of the Pelvis. In the erect posture the pelvis is placed obliquely with 

 regard to the trunk of the body : the bony ring, which forms the brim of the true 

 pelvis, is placed so as to form an angle of about 60 to 65 with the ground on 

 Avbich we stand. The pelvic surface of the symphysis pubis looks upward and 

 backward, the concavity of the sacrum and coccyx downward and forward, the 

 base of the sacrum in well-formed female bodies being nearly four inches above 

 the upper border of the symphysis pubis, and the apex of the coccyx a little 

 more than half an inch above its lower border. In consequence of this obliquity 

 of the pelvis the line of gravity of the head, which passes through the middle of 

 the odontoid process of the axis and through the points of junction of the curves 

 of the vertebral column to the sacro-vertebral angle, descends toward the front 

 of the cavity, so that it bisects a line drawn transversely through the middle of 

 the heads of the thigh-bones. And thus the centre of gravity of the head is 

 placed immediately over the heads of the thigh-bones on which the trunk is 

 supported. 



Axes of the Pelvis (Fig. 125). The plane of the inlet of the true pelvis will 

 be represented by a line drawn from the base of the sacrum to the upper margin of 

 the symphysis pubis. A line carried at right angles with this at its middle Avould 

 correspond at one extremity with the umbilicus, and at the other with the middle 

 of the coccyx : the axis of the inlet is therefore directed downward and backward. 

 The axis of the outlet, produced upward, would touch the base of the sacrum, 

 and is therefore directed downward and forward. The axis of the cavity is 

 curved like the cavity itself: this curve corresponds to the concavity of the sacrum 

 and coccyx, the extremities being indicated by the central points of the inlet and 

 outlet. A knowledge of the direction of these axes serves to explain the course 

 of the foetus in its passage through the pelvis during parturition. It is also 

 important to the surgeon, as indicating the direction of the force required in the 

 removal of calculi from the bladder by the sub-pubic operation, and as determin- 

 ing the direction in which instruments should be used in operations upon the 

 pelvic viscera. 



1 The measurements of the pelvis given above are, I believe, fairly accurate, but different meas- 

 urements are given by various authors, no doubt due in a great measure to differences in the phy- 

 sique and stature of the population from whom the measurements have been taken. The accompany- 

 ing chart has been formulated to show the measurements of the pelvis which are adopted by many 

 obstetr i ci ans. ED. 



A. P. Obi. Tr. 



Inlet 4 4 5 



Cavity 4* 4J- 4 



Outlet 5" 4 4 



