THE

SCIENCE AND PRACTICE

OF

MIDWIFERY.

PART I.

ANATOMY AND PHYSIOLOGY OF THE ORGANS
CONCERNED IN PARTURITION.

CHAPTER I.

ANATOMY OF THE PELVIS.

The pelvis is the bony basin situated between the trunk and the lower extremities. To the obstetrician its study is of paramount importance, for it not only contains, in the unimpregnated state, all the organs connected with the function of reproduction, but through its cavity the foetus has to pass in the process of parturition. An accurate knowledge, therefore, of its anatomical formation may be said to be the very alphabet of obstetrics, without which no one can practise midwifery, either with satisfaction to himself or safety to his patient.

In a treatise on obstetrics, however, any detailed account of the purely descriptive anatomy of the pelvis would be out of place. A knowledge of that must be taken for granted, and it is only necessary to refer to those points which have a more or less direct bearing on the study of its obstetrical relations.

The pelvis is formed of four bones. On either side are the ossa innominata, joined together by the sacrum; to the inferior extremity of the sacrum is attached the coccyx, which is, in fact, its continuation.

The os innominatum (Fig. 1) is an irregularly-shaped bone originally formed of three distinct portions, the ilium, the ischium, and the pubes, which remain separated from each other up to and beyond the period of puberty. They are united at the acetabulum by a Y-shaped cartilaginous junction, which does not, as a rule, become ossified until about the twentieth year. The consequence is that the pelvis, during the period of growth, is subject to the action of various mechanical

influences to a far greater extent than in adult life; and these, as we shall presently see, have an important effect in determining the form of the bones. The external surface and borders of the os innominatum are chiefly of obstetric interest from giving attachment to muscles,

many of which have an important accessory influence on parturition, such as the muscles forming the abdominal wall, which are attached to its crest, and those closing its outlet and forming the perineum, which are attached to the tuberosity of the ischium. On the anterior and posterior extremities of the crest of the ilium are two prominences (the anterior and posterior spinous processes), which are points from which certain measurements are sometimes taken. The internal surface of the upper fan-shaped portion of the os innominatum gives attachment to the iliacus muscle, and contributes to the support of the abdominal contents: along with its fellow of the opposite side it forms the false pelvis. The false is separated from the true pelvis by the ilio-pectineal line, which, with the upper margin of the sacrum, forms the brim of the pelvis. This is of special obstetric importance, as it is the first part of the pelvic cavity through which the child passes, and that in which osseous deformities are most often met with. At one portion of the ilio-pectineal line, corresponding with the junction of ilium and pubes, is situated a prominence which is known as the ilio-pectineal emi

nence.

The internal smooth surface of the innominate bone below the linea ilio-pectinea forms the greater portion of the pelvis proper. In front, with the corresponding portions of the opposite bone, it forms the arch of the pubes, under which the head of the child passes in labor.

Behind this we observe the oval obturator foramen, and below that the tuberosity and spine of the ischium, the latter separating the great and lesser sciatic notches and giving attachment to ligaments of importance. The rough articulating surface posteriorly, by which the junction with the sacrum is effected, may be noted, and above this the

prominence to which the powerful ligaments joining the sacrum and os innominatum are attached.

FIG. 2.

The sacrum (Fig. 2) is a triangular and somewhat spongy bone forming the continuation of the spinal column and binding together the ossa innominata. It is originally composed of five separate portions, analogous to the vertebræ, which ossify and unite about the period of puberty, leaving on its internal surface four prominent ridges at the points of junction. The upper of these is sometimes so well marked as to be mistaken, on vaginal examination, for the promontory of the sacrum itself.

Sacrum and Coccyx.

The base of the sacrum is about 4 inches in width, and its sides rapidly approximate until they nearly meet at its apex, giving the whole bone a triangular or wedge shape. The anterior and posterior surfaces also approximate in the same way, so that the bone is much thicker at the base than at the apex. The sacrum, in the erect position of the body, is directed from above downward and from before backward. At its upper edge it is joined, the lumbosacral cartilage intervening, with the fifth lumbar vertebra. The point. of junction, called the promontory of the sacrum, is of great importance, as on its undue projection many deformities of the brim of the pelvis depend. The anterior surface of the bone is concave and forms the curve of the sacrum, more marked in some cases than in others. There is also more or less concavity from side to side. On it we observe four apertures on each side, the intervertebral foramina, giving exit to nerves. The posterior surface is convex, rough and irregular for the attachment of ligaments and muscles, and showing a ridge of vertical prominences corresponding to the spinous processes of the

vertebræ.

The sacrum is generally described as forming a keystone to the arch constituted by the pelvic bones, and transmitting the weight of the body, in consequence of its wedge-like shape, in a direction which tends to thrust it downward and backward, as if separating the ossa innomi

Dr. Duncan, however, has shown, from a careful consideration of its mechanical relations, that it should rather be regarded as a strong transverse beam curved on its anterior surface, the extremities of which are in contact with the corresponding articular surfaces of the ossa innominata. The weight of the body is thus transmitted to the innominate bones, and through them to the acetabula and the femora (Fig. 3). There counter-pressure is applied, and the result is, as we shall subsequently see, an important modifying influence on the development and shape of the pelvis.

The coccyx (Fig. 2) is composed of four small separate bones, which Researches in Obstetrics, p. 67.

eventually unite into one, but not until late in life. The uppermost of these articulates with the apex of the sacrum. On its posterior surface are two small cornua, which unite with corresponding points at the tip of the sacrum. The bones of the coccyx taper to a point. To it are attached various muscles which have the effect of imparting consider

able mobility. During labor, also, it yields to the mechanical pressure of the presenting part, so as to increase the antero-posterior diameter of the pelvic outlet to the extent of an inch or more.

If, through disease or accident, as sometimes happens, the articular cartilages of the coccyx become prematurely ossified, the enlargement of the pelvic outlet during labor may be prevented, and considerable difficulty may thus arise. This is most apt to happen in aged primiparæ or in women who have followed sedentary occupations; and not infrequently, under such circumstances, the bone fractures under the pressure to which it is subjected by the presenting part.

Pelvic Articulations.-The pelvic bones are firmly joined together by various articulations and ligaments. The latter are arranged so as to complete the canal through which the foetus has to pass, and which is in great part formed by the bones. On its internal surface, where the absence of obstruction is of importance, they are every way smooth; while externally, where strength is the desideratum, they are arranged in larger masses, so as to unite the bones firmly together. The pelvic articulations have been generally described as symphyses or amphiarthrodia―a term which is properly applied to two articulating surfaces united by fibrous tissue in such a way as to prevent any sliding motion. It is certain, however, that this is not the case with the joints of the