Pelvis (Lat., a basin), in anatomy, the lowest of the three great divisions of the trunk, the upper being the thorax and the middle division the abdomen. The term pelvis is generally used to designate the bony framework which supports the lumbar vertebrae and rests upon the thigh bones, while the space included is called the cavity of the pelvis. It is composed of four bones, viz.: two ossa innomi-nata, a a, and, included between them posteriorly, the sacrum, s s, and coccyx, x. Each os innominatum is divided into three parts, the ilium, the ischium, and the pubis. The ilium is the wing or more expanded division marked a a in figs. 1 and 2; the ischium is the lower division, between g and r, the lower part of which, marked t, is called its tuberosity, and is the point upon which the body rests in sitting. The pubis is the anterior division h k These divisions meet in the socket (acetaoulum) of the thigh bone, the dotted line showing the three divisions, which are developed from three primary (eight individual) centres, and which do not become perfectly consolidated till the age of puberty. The sacrum is the wedge-shaped bone s s in the posterior part of the pelvis, between the articular surfaces of the ossa innominata.
Its base is directed upward, having in its middle an oval articular surface corresponding to the articular surfaces of the vertebrae. Behind this is the central canal, which forms the lower portion of the spinal canal. The pelvis is divided by a prominent line ii, fig. 1, called the linea ileo-pectinea, into the false and the true pelvis. The false pelvis is that portion above the line, embraced by most of the ilium, and the upper portion of the ischium. In front the false pelvis is incomplete, and behind there is also a small interval in the middle of the sacrum. The false pelvis is bounded laterally by the crest of the ilium c c, which is a favorable point of attachment for muscles, and its broad wings help to sustain the contents of the abdomen. The true pelvis is all that portion below the linea ileo-pectinea, being mainly embraced by the ischia and pubes. It has a superior circumference, brim, or inlet, and an inferior circumference, or outlet. The brim is formed laterally by the linea ileo-pectinea, anteriorly by the upper margin of the pubes, and posteriorly by the anterior margin of the base of the sacrum, and the sacro-vertebral angle.. (See Skeleton.) The inlet of the pelvis is somewhat heart-shaped, with its obtuse apex in front.
It has three principal diameters, an antero-posterior (sacro-pubic), a transverse, and an oblique. The antero-posterior diameter extends from the sacro-vertebral angle to the symphysis pubis. The transverse diameter extends from the middle of the brim on one side to the same point on the other. The oblique diameter extends from the eacro-iliac symphysis or union on one side to the prominence at the junction of the ileum and the pubes (ileo-pectineal eminence) on the other. The outlet of the pelvis is irregular, and is bounded by three prominent eminences, viz.: the point of the coccyx, x, posteriorly, and the tuberosities of the ischia laterally. These eminences are separated by three notches: one in front between the pubes and the ascending portions (ramus) of the ischia, and called the pubic arch; and one on either side of the sacrum and 'coccyx, called the sacro-sciatic notches. In the natural state the two latter are converted into foramina by the lesser and greater sacro-sciatic ligaments, m n, fig. 3. These ligaments practically lessen the irregularity of the outlet.
The two large openings contained between the rami of the pubes and ischia, and situated below and a little in front of the socket for the head of the thigh bone, are called the obturator foramince, and in the natural state are closed by a ligament called obturator ligament, which gives attachment to muscles of the same name. The diameters of the outlet are two: an antero-posterior, which extends from the tip of the coccyx to the lower part of the symphysis pubis; and a transverse, extending from the posterior part of one ischiatic tuberosity to the same point on the opposite side. The cavity of the true pelvis is contained between the inlet and the outlet, and has an average depth posteriorly (o x, figs. 1 and 2) of 4.5 in., being the length of the sacrum and coccyx. Its depth at the middle is 3.5 in., and anteriorly, at the symphysis pubis, it is 1.5 in. The diameters of both the brim and the outlet differ in the two sexes, as do the characteristics of the whole pelvis. The average measurement of the diameters in an adult female pelvis (fig. 2) are as follows: brim - antero-posterior, 4.4 in.; transverse, 5'4 in.; oblique, 4.8 in.; outlet - anteroposterior, 5 in.; transverse, 4.3 in.
The soft parts, composed of various tissues, practically reduce these diameters from about one half to one inch or a little more. In the male pelvis (fig. 1) the bones are thicker and stronger, the eminences and impressions on their surfaces for the attachment of muscles are more strongly marked, the cavity is deeper and narrower, and the obturator foraminae larger. The female bones are lighter, more expanded, and less massive, the sacrum is wider and less curved, the promontory less projecting, and the coccyx more movable; the arch of the pubes is also wider and the edges are more turned outward; the tuberosities of the ischia and the acetabula are wider apart, and the ilia are wider and more expanded. For several years after birth the human pelvis is small in proportion to that of the adult; at the commencement of puberty the pelves of the two sexes are much alike, but at that period they begin to develop more rapidly and to acquire their distinguishing characteristics. The pelvis differs somewhat in the different races of the human family, and much more in the lower mammalia. In the negro it is longer and narrower* the difference being greater in the male than in the female.
In the apes and monkeys the difference greatly increases, as shown in fig. 4, which represents the pelvis of a large monkey where the transverse diameter was 2.7 in., and the antero-posterior 3 in. As the dimensions of the pelvis are closely related to the act of parturition, the reduction in the diameters and the increased length and straightness of the cavity in the negro are compensated by the smaller size of the cranium of the offspring. The cavity of the pelvis in ordinary conditions of the individual contains the urinary bladder and internal organs of generation, and also the lower portion of the intestinal canal. There is no membranous division between the cavities of the abdomen and pelvis, as there is between the abdomen and the thorax (see Diaphragm), and sometimes the pelvic cavity is spoken of as a part of the general abdominal cavity.
Fig. 1. - Male Pelvis, European.
Fig. 2. - Female Pelvis, European.
Fig. 8. - Side View of Eight Os Innominatum.