AXES OF THE PELVIS. - The term axis is applied anatomically to the line of direction of any surface or plane, and, as it implies a right line, drawn at right angles to that surface or plane, it can only be applied with propriety to the outlets of the pelvis. As applied by some authors to the line which indicates 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 nunaber 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 fenzale 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. l, ni). 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 l). 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 7.5°, 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 (n, 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 rnotion 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 an, s, this axis is thrown niore 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 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 az). 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 areArcd, from the immobility of the pelvic walls which include them. But, below the inferior tlotted plane to that of the outlet, the axis is di rected more forwards (n, q), as the coccyx moves backward in the curve (in, s), a de viation which facilitates the exit of the fcetus in parturition. This forward 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 axis of the pelvic ca vity is not one right line, as stated by Mi ler and Ilcederer ; nor is it properly ex pressed by perpendicular lines drawn from three planes, as Levret suggested ; nor by a continuous geometric " arc de cercle," 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 inferior 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.