CHAPTER I.

OF THE PELVIS.

The formative organs of generation are situated within a large cavity, called the cavity of the pelvis, the walls of which are composed of bones and soft parts. This basin (in Latin, _pelvis_) is an irregular, long cavity, situated at the base of the spinal column, and above the inferior extremities. In the adult the bony pelvis may be divided into four parts or bones, viz: the os sacrum, two ossa innominata, and the os coccygis, but in early life they are more minutely divisible.

THE SACRUM.

The sacrum (Fig. 1) terminates the vertebral column, and is perhaps the most important bone in the pelvis, obstetrically considered, as it enters largely into the various deformities of the pelvis. In the adult it is of a triangular shape, the base of the triangle being above and inclining forwards, the apex below and somewhat backwards; its length is from four to four and a half inches; its breadth about four inches, and the greatest thickness, two and a half inches. The internal surface is concave to the amount of half an inch, crossed by four transverse lines, marking the former division by cartilage; here are four pair of holes, through which pass numerous nervous filaments, which afterwards form part of the great sciatic nerve.

Fig. 1– A represents the internal or anterior surface of the sacrum. B B represents the articular processes. C C represents the anterior sacral foramen. D represents the articulating surface.

It is placed at the posterior part of the pelvis, where it appears like a wedge forced in between the ossa innominata, immediately below the vertebral column and directly above the coccyx.

THE OSSA INNOMINATA.

The os innominata (nameless bone, Fig. 2) is of a very irregular figure, and the pair occupy the lateral and anterior parts of the pelvis. The external or femoral surface is turned backwards and downwards, as well as outward; at its superior part, inferiorly it looks downwards. Towards the front, the external face presents the cotyloid cavity, or the acetabulum; a little more in advance and below is the subpubic or obturator foramen, which is nearly closed by the obturator ligament.

The superior portion of the bone is characterized on its abdominal or internal face by a large excavation called the internal iliac fossa (Fig. 4.) This portion is terminated below by a large rounded and concave line. The inferior (lower) portion presents behind a nearly triangular plane surface; near the middle of this is the obturator foramen, and in front is the internal face of the os pubis.

THE OS COCCYGIS.

The OS COCCYGIS (Fig. 5) is three or four little bones united together on the median line of the body, and attached to the os sacrum. Each little bone is tipped with cartilage, and they are so united as to be movable. The entire bones form a pyramid, the apex of which is below. The internal surface is smooth, like that of the sacrum, terminating the plane of the sacrum and bounding it anteriorly.

Of the JOINTS OF THE PELVIS it is only necessary here to say that there is no motion in them to facilitate labor, except that the sacro-coccygeal joint is of the kind called ginglymoid, admitting of extensive motion, especially backward, so as to permit the enlargement of the lower outlet an inch or more. (Fig. 6.)

OF THE PELVIS IN GENERAL.

We will now consider the pelvis collectively or as a whole; its relation to the rest of the body; its magnitude, axis, etc. It is connected with the trunk by the articulation of the sacrum with the last lumber vertebra, effected in the same manner as the junction of the vertebra with each other; with the lower extremities it is connected by means of the hip joints. When the pelvis is _in situ_, the brim is neither horizontal nor perpendicular. It represents a cone, slightly flattened from before backwards, the base of which being above, while the apex is directed downwards.

When the body is erect the upper part of the sacrum and the acetabula are nearly on the same descending line, the point of the os coccygis being a little above the arch of the pubis, and the sacro-vertebral angle three inches and nine lines higher than the pubis. Were it not for the obliquity owing to the upright position of the human female, the womb would gravitate low in the pelvis, and produce most injurious pressure on the contained viscera. The lower or true pelvis is the part involved in parturition, and its size and shape demands our attention.

THE BRIM OF THE PELVIS.

This is defined by the LINEO ILIO PECTINEA, which marks the boundary of the true and false pelvis, and this _superior strait_ is the entrance of the lesser pelvis. Its form has been variously described as being oval, heart-shaped, and triangular. If we call it “triangular with angles rounded off,” the base of the triangle is behind and the apex in front. It would be nearly oval were not the oval form broken by the promontory of the sacrum. This brim is the first solid resistance the head of the fœtus meets in its descent through the pelvis.

DIAMETER OF THE PELVIS.

Different estimates are made by different anatomists of the measurements of the brim of the pelvis. The following is nearly the correct size of the ordinary female pelvis:

The circumference varies from thirteen to fifteen inches; the antero-posterior diameter, _i. e._, from the prominence of the sacrum to the upper edge of the symphasis pubis, (Fig. 6), is about four and a quarter inches; the transverse across the widest part of the brim, at right angles to the antero-posterior, is five and a quarter inches, and the oblique from the sacro-iliac synchondrosis of one side to the opposite of the brim, just above the acetabulum, is five inches. (Fig. 7).

THE CAVITY OF THE PELVIS, of which the fixed boundaries are the sacrum and the pubis, is of unequal depth. The height in front is one and a half inches; upon the sides, three and three-quarter inches, and it is four and a quarter inches if a straight line be drawn from the sacro-vertebral angle to the point of the coccyx, five and a quarter inches following the curve of the sacrum, and six inches if the coccyx be extended. (Fig. 6).

The antero-posterior diameter of the outlet from the arch of the pubis to the point of the coccyx is usually four and a quarter inches, but may increase to five inches during labor by the retrocession of the coccyx (Fig. 8); the transverse from one tuber ischii to another is four and a quarter inches, and the oblique about four and three-quarter inches.

It is important to notice that the diameters are entirely changed between the rim and the outlet, and that the change is effected gradually. The axes of the inlet and outlet form an obtuse angle with each other (this is illustrated in Figs. 6 and 8.) The three diameters taken at the center of the pelvis are very nearly equal—about four and three-quarter inches.

DIFFERENCES OF THE PELVES.

There is considerable difference between the male and female pelvis, in shape and size. The pelvis in the male is smaller but deeper; the bones are thicker and the brim is more circular, the depth of the symphasis pubis is greater, the sacrum is more perpendicular, the arch of the pubis is narrower, the tuber ischii are nearer each other, and the coccyx less movable. In the female the iliac fossæ are larger, the interval separating the angle of the pubis from the acetabulum is greater, causing the prominence of the hips and wider separation of the thighs, the superior straight is larger and more elliptical, the curve of the sacrum deeper and more regular, the tuberosities of the ischii are further apart, and the arch of the pubis broader. From the greater width of the female pelvis, and from the upper end of the thigh bones being farther apart than in the male, the thigh bones approach each other in their descent, giving a peculiarity to the movements of the female in walking.

The soft parts lining the pelvis and covering it externally modify the diameters of the pelvis, but the effect of these additions in diminishing the internal diameter is not very great. The diameter of the cavity is lessened thereby from one-fourth to one-half an inch.

USES OF THE PELVIS.

One function of the pelvis is to inclose and protect the bladder, rectum and seminal vesicles of the male, the uterus, Fallopian tubes and ovaries, as well as the bladder and rectum in the female. During labor it affords a passage for the child.

TERMINAL OUTLET OF THE PELVIC CANAL.

This is not at the coccyx, but rather at the anterior commissure of the perineum. This is so greatly distended at the last moment of labor as to much prolong the posterior wall of the pelvic excavation and the canal to be traversed by the fœtus. (Fig. 8).