THE study of the bony skeleton of the pelvis forms the only true basis for understanding this most important part of the body, as it appears in real life. And it is only now, as we come to the examination of the pelvis, clothed with muscular and other living tissues and supplied with organs that its real study begins. Occupying the middle ground between the upper and the lower part of the frame, the pelvis affords attachment to two classes of muscles in addition to those peculiar to itself and employed in its own uses.
The first class is for completing, enclosing and perfecting the abdomen. The muscles of this class, by their great power of distension, afford every needful facility for the development and support of the foetus in the pregnant state. These muscles, which form the anterior abdominal parietes, by yielding to the pressure exerted from against the rigid posterior walls, cause the gravid uterus to project over the arch of the pubis, and thus hinder it from settling downward through the superior strait.
Fig. 6.
A. A section of the aorta. B. The vena cava inferior. C. The internal iliac artery arising together with D, the external iliac, from the primitive iliac trunk. E. External iliac vein. F. The iliacus internus, and G, the psoas magnus muscles. H. The rectum. 1. The uterus with its appendages. K. The bladder, the fundus of which is depressed so as to bring the womb into view.
The muscles of the second class are those which, connecting the lower extremities, are concerned in locomotion. There are but four muscles on the inner surface of the pelvis. The principal of these are the iliacus internus and psoas magnus; next in size and relative importance come the pyriformis and obturator muscles.
The iliacus and psoas magnus, while being themselves protected by the iliac fascia, serve to line and cushion the iliac fossae and bones of the upper pelvis thus preventing the uterus from receiving injury in the later months of gestation, from the shocks and concussions inseparable from active exercise. These two muscles, as they become conjoined in passing over the lateral parts of the superior strait to be inserted into the lesser trochanter, have the effect to change the base of the curvilinear triangle of the pelvis from the rear to the front, and to shorten the transverse diameter half an inch; these two being the only muscles in the pelvis that do shorten any of its diameters. The iliacus, however, from its thinness, neither too much encroaches upon the concavity of the iliac fossae, nor indeed does, it, so much as the psoas diminish the size of the pelvic cavity. And the shortening of the transverse diameter may be in a great measure obviated by flexing the thighs upon the abdomen in labor, thus relaxing the psoas muscles and reducing the diminution of the pelvic cavity to the smallest possible amount.
The pyriformis, arising principally from each outer margin of the sacrum, passes immediately out of the pelvis through the great sacrosciatic foramen, to be inserted into the great trochanter of the femur. The obturator internus muscles arise from the inner surface of the obturator membrane and adjacent parts, and leaving the pelvis through the lesser sacro-sciatic foramen, are also inserted into the upper border of the great trochanter, near the pyriformis. Neither of these muscles serve to diminish the diameter of the pelvis.
Thus we have the entire cavity of the pelvis covered by fascia and sufficiently supplied with muscular tissue for all its uses, without so much diminishing its size as to obstruct the child in its descent. The obturator internus and pyriformis, covering the obturator and sciatic foramina, rather allow increase of room for the foetal head than decrease the pelvic excavation. So the coccygeal muscles, very thin and connected with the sacro- sciatic ligaments, do not encroach laterally upon the cavity of the pelvis. In like manner the pyriformis muscles close the sacral foramina in the posterior of the pelvis, but do not diminish its size. The anterior posterior diameter is slightly diminished by the attachment of the bladder to the posterior surface of the arch of the pubis. And the cellular tissue, which every where lines the pelvic cavity in fleshy females, is liable to become loaded with fat,-which would offer serious obstruction in parturition. The rectum, in passing down over the anterior surface of the sacrum and to the left of its promontory, does not materially hinder the process of labor, unless loaded with indurated fecal matter. While the uterus, although occupying a middle ground between the bladder and rectum, does not shorten any diameter of the pelvis in labor, – for at such times it is usually entirely above the superior strait.
The Perineum forms the external concavity and internal cavity of the floor of the pelvis, and is composed of two distinct planes or layers of muscles. The upper layer is composed of the levator ani and coccygeal muscles, and forms the concave floor which sustains the vesicles of the pelvic of the pelvic cavity. The lower plane consists of the sphincter ani, the transversus perinei, the ischia-cavernosus and the constrictor vaginae muscles; and has its concavity looking downward. This combined muscular arrangement constitutes the floor of the pelvis, which is completed by the internal pubic vessels and nerves, a large amount of cellular tissue, the inter-muscular and pelvic aponeuroses and skin.
Fig. 7.
Positions of the pelvis and the direction of its axis in the dorsal attitude assumed by the female during labor.
a.b. Total axis of the excavation, being a continuation of d, b, the axis of the superior strait c.v. Perineum as distended at the moment of the passage of the head. r. Anal orifice. e.v. Terminal plane of the of the pelvis.
The perineum extends one inch and three-fourths from the point of the coccyx to the anus; thence one inch and one-fourth to its anterior terminus in the vulva, – making three inches in all in its ordinary condition. But at the instant of the passage of the child’s head into the external world at full term, the whole floor of the pelvis becomes so distended downwards and forwards as to cause the perineum to measure five inches and three- fourths.
It will now be seen that the terminal outlet of the pelvic cavity in parturition, is not directly at the inferior strait,- but at the point sufficiently beyond, so as to allow the pelvic floor to become so distended as to render it possible for the child’s head to pass at the same time underneath and beyond the symphysis pubis. Thus, in passing through the living pelvic cavity, the child must enter the superior strait and take its course, relatively to the mother, at first downwards and backwards, then downwards and forwards till it passes beneath and beyond the symphysis pubis,-exactly following the axis of the entire cavity, which is a regular curve.
