PELVIS. 



135 



the central point in any given plane of the 

 pelvic cavity, it then becomes really a curved 

 line, made up of an infinite number of perpen- 

 diculars drawn from any number of planes 

 radiating from a centre placed anterior to the 

 symphysis pubis. Since it is with regard to 

 the mechanism of parturition principally that 

 the axes of the pelvis are of importance, the 

 angles formed by them, with the vertebral 

 plane (transverse vertical), are stated in refer- 

 ence to the standard female pelvis more par- 

 ticularly. In the male subject, these angles 

 will be somewhat greater, from the greater 

 inclination of the pelvis in that sex. 



The axis of the brim is a line drawn from the 

 centre of the superior plane, and at right 

 angles to it (fig. 84. /, w). This line cuts the 

 prolonged vertebral plane exactly half-way 

 between the symphysis pubis and the upper 

 part of the third sacral vertebra, and forms 

 with it an angle of about 60 (a o /). It may 

 be taken also as the most nearly approxima- 

 tive axis of the pelvic cavity above that point. 

 When prolonged at each end, it passes out 

 at the umbilicus, and impinges upon one of 

 the two last coccygeal bones, in a well-formed 

 female. It is evident, however, that from the, 

 great variety of the sacro-coccygeal curves, 

 that the point where this line meets the 

 coccyx will be variable. Hence, the observa- 

 tion of Watt, that a line joining the tip of the 

 coccyx and the centre of the superior plane 

 cuts the latter at an angle of 75, is too defi- 

 nitive. M. Naegele, however, found that in 

 a large number of female pelves, this line 

 did meet the coccyx at some point or other. 



The axis of the inferior outlet (w, p) is 

 drawn at right angles to the centre of the in- 

 ferior plane, and falls midway between the 

 sciatic tuberosities. From the mobility of the 

 coccyx, it will vary with the motion of that 

 bone from its ordinary position to a position 

 of extreme extension. In the ordinary posi- 

 tion of the coccyx, this axis forms, with the 

 vertebral plane, an angle of about 10, and 

 meets it near the centre of the upper surface 

 of the body of the first sacral vertebra, im- 

 pinging there upon the sacral promontory. 

 When the coccyx is in a position of extreme 

 extension, however, its tip describes the 

 curve ?, *, this axis is thrown more forward 

 (n, q\ and forms a less angle with the verte- 

 bral plane ; while the plane of the inferior 

 outlet itself is depressed (g, s), and its angle 

 with the horizon (s g d] diminished. 



The curved line (/, o, n, p), which indicates 

 the continued centres of the planes of the 

 pelvic cavity, may be divided into three por- 

 tions indicated in the figure by the two dotted 

 planes (c, h and c, r). The part from the 

 plane of the outlet to the upper dotted plane 

 (c, h}, impinging upon the third sacral ver- 

 tebra, may be considered to coincide, for all 

 practical purposes, with the line of axis of the 

 brim (/, m). The inferior dotted plane (c, r), 

 drawn, like the former, from the point of junc- 

 tion of the planes of the brim and inferior 

 outlet, to the tip of the last sacral vertebra, 

 includes, with the upper dotted plane just 



mentioned, a parabolic curve (o, n), which does 

 not quite coincide with the arc of a circle 

 drawn from the ante-pubic centre (c). These 

 two portions of the axes of the cavity arefaed, 

 from the immobility of the pelvic walls which 

 include them. But, below the inferior dotted 

 plane to that of the outlet, the axis is di- 

 rected more forwards (n, <?), as the coccyx 

 moves backward in the curve (?, s), a de- 

 viation which facilitates the exit of the foetus 

 in parturition. This forwaid direction of the 

 axis at this part is increased also by the 

 rounding off of the symphysis pubis at its 

 inferior border. Each of the three portions 

 passes midway between the corresponding and 

 opposing surfaces of the symphysis pubis 

 and sacro-coccygeal wall, the general vertical 

 outline of which it nearly resembles, and be- 

 tween the lateral ilio-ischial columns. The 

 latter, being equally inclined to each other 

 downwards, cause no deviations in the plane 

 which forms the centre of their lateral dis- 

 tances. 



So that the so-called a.vis of the pelvic ca- 

 vity is not one right line, as stated by Miil- 

 ler and Koederer ; nor is it properly ex- 

 pressed by perpendicular lines drawn from 

 three planes, as Levret suggested ; nor by a 

 continuous geometric " arc decercle" from the 

 superior to the inferior plane, as G. Bang, 

 Choulaut, Camper, and Carus concluded; 

 nor by the meeting of the axes of the superior 

 and inferior outlet, as is somewhat loosely 

 stated by some more modern writers on ob- 

 stetrics ; but it is a more or less irregular 

 parabolic curve, passing from the fixed axis 

 of the brim, and moveable forwards at its 

 inferior extremity with the moveable axis of 

 the inferior outlet, with which it coincides 

 below. 



It may be here added, to prevent mis- 

 conception, that the line of direction of the 

 interior outlet is in the living female inclined 

 forwards in a much greater degree (n, b"), than 

 that of the osseous pelvis, by the prolongation 

 of the posterior wall in the soft parts of the 

 perineum. 



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 A to 

 A 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 projecting, 

 while the pubic arch is narrow and angular, 



K 4 



