In the superior or abdominal pelvis, the anterior region is formed in the living subject by the muscular parietes of the abdomen. The posterior presents the sacro-vertebral symphysis and promontory, and extends to the sacro-iliac symphysis on either side. Each lateral region is constituted by the internal iliac fossa, and covered by the iliacus internus and psoas muscles.
In the lower or smaller pelvis, the anterior region is composed of the posterior region consists of the anterior surface of the sacrum and coccyx. Each lateral region of this division of the pelvis, may be again divided into an anterior and a posterior portion. The anterior portion is composed of the bony structure of this part of the pelvis, and corresponds to the inner surface or back part of the cotyloid cavity, and to the interior surface of the body and tuberosity of the ischium. The posterior portion is represented by the greater and lesser sacro-sciatic ligaments, and by the inner surface of the greater and lesser sciatic notches, which, by these ligaments, are converted into foramina and pelvic walls.
THE INCLINED PLANES OF THE PELVIS.-Inside the pelvis are found four inclined planes,- one anterior and one posterior on each side. These inclined planes may be demonstrated by dividing the pelvis itself into two lateral halves by vertical section through the symphysis pubis and median line of the sacrum; and by again dividing each of these lateral portions into anterior and posterior quarters by a transverse vertical section, on the ilio- pectineal line, a little in front of the sacro-iliac symphysis, and terminating in the spine of the ischium.
Each of the four-quarters thus formed of the pelvis, will be found to represent an inclined plane. And each of these inclined planes will be found to look in the same general direction. Thus each of the anterior inclined planes-that is the right and left anterior, will be seen to look from within, outwards; from above, downwards; and from behind, forwards. In like manner both the right and the left posterior inclined planes will look from without, inwards; from above, downwards; and from before, backwards. It was formerly supposed that these several inclined planes, by influencing the rotation of the head of the foetus, through successive changes of direction, exerted a powerful influence on the mechanism of labor. But, in point of fact, the rotation occurs principally after the head has so far passed through the inferior strait, as to engage the soft parts and distend the perineum. And it may be now stated that these inclined planes serve in part to prevent the too rapid descent of the head, and in part to cause it to assume and maintain the most natural and favorable position in its course.
In its general appearance, the interior of the pelvis may be said to resemble the inner surface of an irregularly shaped and truncated cone, tapering downwards. Although from the strongly marked depression of the hollow of the sacrum, the cavity of the pelvis seems larger than either the entrance above or the outlet below. The ilio-pectineal line, – already described as dividing the greater from the lesser pelvis,- constitutes also what is called the margin of the superior strait. And the corresponding border of the apex or inferior outlet of the pelvis, constitutes what is known as the inferior strait. The engagement of the head in the former, or superior strait, and is subsequent emergence through the latter, or inferior strait, form the most important events in the process of parturition. Hence the axes and dimensions, and in fact all the parts and relations of these straits should be thoroughly understood. The superior strait corresponds to what is by some termed the brim, as the inferior strait corresponds to the outlet; while the intervening internal body of the pelvis is indifferently named its cavity or excavation. These three make up the entire obstetric canal of the pelvis.
THE SUPERIOR STRAIT.- The Superior Strait, or brim of the pelvis, presents the shape of a curvilinear triangle with rounded angles.- having its base behind and its apex in front. The sacro- vertebral promontory, jutting in, as it were, gives a heart- shaped appearance to what might otherwise have been nearly oval. The psoas muscle on either side, by diminishing the transverse diameter, causes the superior strait to assume a more triangular form in the living subject than it presents in the bony skeleton. The boundaries of the superior strait are, the sacro-vertebral angle, the anterior border of the wings of the sacrum, the linea ilio pectinea, the horizontal rami, and superior margin of the symphysis pubis.
The very great obstetric importance of this superior strait arises from the fact that it forms the first part of that bony canal through which the foetal head must be transmitted, and through the somewhat contracted opening of which it must pass, almost before it acquires any of that elongated shape, which it so often presents on emerging subsequently from the outlet or inferior strait.
Fig. 1.
c. The plane of the superior strait prolonged beyond the pubis. c.e. The plane of the inferior strait prolonged beyond the pubis, c.d. Shows the departure of this plane from the horizontal line. a.b. The axis of the superior strait. g.f. The axis of the inferior strait.
The plane of the superior strait, its axis and its dimensions form the most important points for considerations respecting it. The plane or surface of the superior strait, may readily be demonstrated, by cutting a piece of paper to fit, it making it extend antero-posteriorly from the sacral promontory to the pubic arch; and transversely from the ilio-pectineal line of one side to that of the other. And when the female is in the erect position, it will be found that this plane presents an inclination of from fifty-five to sixty degrees to the horizon.
The axis of this plane will of course, be a line which shall fall at right angles upon its centre. Thus the axis of the superior strait must form the same angle with the vertical line, that the plane of the strait forms with the horizontal line. And upon examination it is found that this axis extends from a point on the linea alba a little below the umbilicus to the articulation of the second bone of the os coccyx with the third. This portion of the coccyx is therefore opposite or parallel to the plane of the superior strait. The inclination of the plane of the superior strait, and of course the directions of its axis, must vary with the changing position of the female. The angle being diminished when the trunk is bent forward, as in stooping over.,-and becoming least in the recumbent position. But it is greatly increased when, in the advanced stages of pregnancy, the female straightens up and even leans backward as it were, in order to maintain her equilibrium. So in parturition, as will be more particularly explained subsequently, the position and flexure of the trunk, when standing or lying down, will greatly affect the entrance of the head into and its passage through the superior strait.
In the dimensions of the superior strait, we note its different diameters and its circumference. The irregular shape of the figure of this strait renders its various diameters unequal. Different authors have enumerated quite a number; but the three principal diameters are all that are really or practical importance. These are, first, the antero-posterior diameter, which extends from the sacro-vertebral angle to the superior border of the symphysis pubis. This, which is also called the sacro-pubic, or conjugate diameter, measures four and a half inches. The second, the transverse diameter, or long diameter, extends across the pelvis at its widest part, and at right angles with the antero-posterior diameter. This, which is also called the iliac or lateral diameter, -since it extends from the middle of the rounded border of the iliac fossa of one side to that of the other,-measures five inches. The third is the oblique or diagonal diameter. This extends from the sacro-iliac symphysis to the ramus of the pubis, or more exactly speaking, to the ilio-pectineal eminence of the opposite side; and measures four inches and three -fourths. In consequence of the space occupied by the soft parts, these dimensions are considerably less in the living body. And we must allow at least one-quarter of an inch in the antero-posterior diameter; and one half an inch in the transverse diameter.
The circumference of the superior strait,-bounded anteriorly by the inner margin of the horizontal rami of the pubis, laterally by the ilio -pectineal line, posteriorly by the promontory of the sacrum, -measures from thirteen and a half to seventeen inches.
THE INFERIOR STRAIT.-The Inferior Strait is smaller than the superior; and in the bony skeleton much more irregular in form. This latter circumstances arises from the projection of the coccyx behind, and of the tuberosities of the ischia on each side. These projections form three deep notches, one on each side of the coccyx, and one immediately beneath the symphysis pubis. Thus this irregular border or periphery of the inferior strait is made up by the symphysis and descending rami of the pubis, by the tuberosities and ascending rami of the ischia, by the lower border of the posterior sacro-sciatic ligaments and by the extremity of the coccyx which projects inward. In parturition, however, the coccyx is usually pushed back, rendering the actual obstetric outlet nearly oval. This oval figure of the outlet may be demonstrated by placing the apex of the pelvis on a sheet of paper and tracing its outline with a pencil. And the figure thus obtained, compared with that of the brim of the pelvis, will show a gradual inclination of the walls of the pelvis downwards and inwards.
