The common plane, and axis of the pelvic cavity ought to be well understood, as they represent the gradual change from the primary plane and axis of the superior strait above, to the final plane and axis of the inferior strait beneath.
Owing to the concavity of the sacrum, and to the slight convexity of the posterior surface of the pubic wall, the passage through the pelvic excavation represents a curve, which if continued would produce a circle. Consequently the plane which commences at the superior strait, is constantly changing as we pass from above downwards. And as the plane changes, so also must the axis of the plane, to correspond to it. Thus the general plane of the excavation will be represented only by a succession of differently inclined planes. So a corresponding series of axes must represent the general axis of the excavation.
Fig. 5.
a b. The plane of the superior strait. i.o. The plane of the inferior strait. c. The point where these two planes would meet, if prolonged. m.n. The horizontal line. e.f. The axis of the superior strait. g.k. The axis of the excavation. p q r s t. Various points taken on the sacrum to show the plane of the excavation at each point.
This general series of planes, and corresponding series of axes, is well illustrated in Fig. 5, – thus a b represent the plane of the superior strait; i.o, the plane of the inferior strait; c, the point of union of these two planes, if prolonged; then p q r s t-c, will represent the successive planes which must result from the curvature of the sacrum. It being borne in mind that the axis must always be regarded as perpendicular to (that is at right angles with ) its plane, the axis of the first plane, or that of the superior strait will therefore be seen to be at right angles with its plane. And since the sacrum at this upper part of the pelvis forms a straight line, so the plane of the superior strait will from a right angle with it,- and the axis of this plane will consequently be parallel with the sacrum. And so in fact in each successive instance; the plane of each successive point of the cavity, must be at right angles with the opposite point of the sacrum; and the axis of each successive plane must be at right angles with the plane itself and of course parallel with the corresponding portion of the sacrum. These successive axes, thrown into a single line, will therefore form a curve exactly corresponding to the opposite curve of the sacrum. This general axis of the excavation, g, k, represents the line of direction which the foetal head must necessarily pursue in its course through the canal of the pelvis. And as this line in a geometrical point of view, is the result of a union of successive lines, -so in a physiological point of view, the course followed by the foetal head is the result of a combination of various and successive vital forces, whose progressive development can take place only in the direction rendered possible by the form and construction of the internal surfaces of the pelvic cavity.
THE DIFFERENCES BETWEEN THE MALE AND FEMALE PELVIS.-
These differences are quite numerous and very strongly marked. The pelvis of the male is smaller, stronger, narrower, deeper in its cavity, and more circular in its brim. The symphysis pubis is nearly double the depth of that of the female; the sacrum is straighter and the sacro-sciatic notches and foramina smaller; the obturator foramen more oval; the pubic arch is straight, much narrower, triangular in shape; its walls and the tuberosities of the ischia are less widely separated; the coccyx is more immovably attached to the sacrum; and in general the articulation are sooner anchylosed than in the female.
The pelvis of the female is much wider; the acetabula are further apart, although the knees approach each other even more than in the male, giving to the movement and gait of the female some distinguishing peculiarities. The superior strait is larger and more elliptical; the curve of the sacrum is deeper and more regular; the symphysis pubis is but half as deep as in the male; and the pubic arch, is broader, more rounded and its lateral walls more widely divided. The entire pelvis is larger and more capacious, and so constructed, in its several arches and ligaments, as to combine the greatest possible lightness, elasticity and strength, with the most suitable form and dimensions for sustaining the gravid uterus above its brim, or enabling the full grown foetus to be transmitted through its canal.
In early childhood the pelvis seems disproportionably small, and even incapable of containing all the organs which occupy it in after life. It is narrow and long; and the abdominal protuberance which is seen in the foetus and in newly born infants, and even still later in the case of rachitic children, is due to its thus forcing upward some of the parts which should be contained within it. But, as the female child advances in life, the pelvis becomes gradually, although slowly enlarged, till, at the period of puberty, it experiences, within a short time, an alteration so extensive and so strongly marked, as to change perceptibly the form and contour of the hips. At nine years of age, the antero- posterior diameter measure two inches and seven-eighths, and transverse diameter, two and three-fourths; while at fourteen years of age, the antero-posterior diameter measures three inches and three-fourths, and the transverse, four and a half.
THE USES OF THE PELVIS.-The pelvis of the female, as in the male, affords a general support to the abdomen and its contents. In the pregnant female, it especially supports the gravid uterus upon its anterior arch and iliac wings. Within the cavity of the pelvis, the uterus of the unimpregnated female, the ovaries and Fallopian tubes are safely enclosed; while the delicate and sensitive external organs of generation are securely protected beneath. From its relation to the lower portion of the spinal column, and from the manner in which it is place upon its femoral supporters, the pelvis may be regarded as the structural base or foundation of the human body. While, from the study of the functions which it performs in connection with the organs which it contains, it will no less obviously be seen to be the true physiological centre of the entire human system. From improper management in childhood and youth, from too long and too frequent indulgence in unnatural positions, from habitual over-exertion of a particular kind from accidental injuries, and especially from disease and softening of the bones, the pelvis may become contracted, strained and distorted, in every direction, – and so more or less incapable of performing its most important uses in the support of the body and in parturition. So also, from similar and corresponding causes, the organs which it contains may become more or less disordered and incapable of performing their vital functions. In the former case, mechanical difficulties and obstructions embarrass the accoucheur. The consideration of these difficulties, and of the appropriate means for overcoming them, we reserve as a sequel to the normal process of parturition. In the latter case the physician has to contend with various forms of functional disorder or structural disease. The consideration of these disorders, and of their appropriate remedies, we shall take up as we come, in course, to the examination of the organs of which they are affections.
