Each os innominatuni articulates with its fellow of the opposite side (through the intervention of the interosseous fibrozartilage, which unites the two surfaces of lie pubic bones, see Fig. ILA with the serum, and with ihe femur (at the aeet:111111m). No less than 85 muscles are atta•hed to this hone, some proCeeding to the region of the hack, others farming the wails of the abdomen, others forming the floor of the pelvis, others passirig downward to the lower ext hies, As the other bones entering into the formation of the pelvis, the sacrum, and the coccyx, belong essentially to the vertebral column, and will be described in the article on that subject, it is sufficient here to remark that, collectively, they form a triangular bony mass (with the base upward, and with a con cave anterior surface), which constitutes the posterior part of the pelvic ring. See Fig. 11. 4, 5.
The pelvis, considered as a whole, is divisible into a false and true pelvis. The false pelvis is all that expanded portion which is bounded laterally by the iliac hones, and lies above the prominent line term( d the Ultra ileo-feetinta (see Fig. II. k, /); while the tree pelvis is all that part of the general pelvic cavity which is situated below that line. The broad, shallow cavity of OW false pelvis serves to support theweight of the intestines; while the rectum, bladder, and part of the generatiVe organs, lie in the cavity of the true pelvis. Tte upper aperture of the true pelvis is termed the inlet. It is somewhat heart shaped in form, and has threeprincipal antero-posterior (or micro-pubic). which extends from the angle formed by the sacrum with the last lumbar vertebra to the symphysis pubis, or point of union of the two pubic bones; the transverse, at right tingles to the former, and extending across the greatest width of the pelvis; and the oblique, extending from the sacroiliac symphysis (or union), on one side, to the margin of the brim corresponding with the neetnbulum on the other. The diameters of the out let are two—an a ntern-posterior, extending from the tip of the coccyx to the lower part of the symphysis pubis; and a transverse, from the posterior part of or ischiatic tuber osity, to the same point on the opposite side. As the precise knowledge ot the diameter and depth of the pelvis is of the greatest importance in the practice of midwifery, we give the average numbers representing the dimensions of a well-formed adult female pelvis. Diameters of inlet or brim—witero-posterior, 4.4 in.; transverse, 5.4 in.; oblique. 4 8 inches. Diameters 4' outlet—antero-posterior, 5 in. ; transverse, 4.3 inches. Depth of the trite pelth—posieriorly. 4.5 in, ; in the middle, 3.5 in.; anteriorly, 1.5 inches.
-he pelvis is placed obliquely with regard to the trunk of the body; the plane of the inlet to the true pelvis forming an angle of from 00° to 65° with the horizon. According Naegele (Veber dos weibliehe Beekeit), the extremity of the coccyx is in the female, when standing upright, about 7 lines higher than the lower edge of the symphysis pubis: the tipper edge of the symphysis being at the same level as the lower edge of the second !segment of the coccyx. By attention to these data:, a detached pelvis may readily be placed at the angle at which it normally lies in the skeleton. The shape of the human pelvis is much affected by the curving forward of the lower part of the sacrum. This bend of the sacrum forward serves to support the viscera, when the body is in an erect posturp; but it is of much more importance in its relation to the act of parturition. If all the antero-posterior diameters of the true pelvis from the brim to the outlet were bisected, the points of bisection would form a curved line, similar to the curve of the sacrum, and termed the axis of the pelvis. As the head of the child bas to follow this
curve, the difficulties of parturition are much greater than if the axis of the pelvis had been straight, as in the other vertebrate. Without entering into unnecessary details, we may remark generally, that the fatal head is of oval shape, with its greatest diameter from before backward, and that in its passage through the pelvis it is so placed that its longest diameter at each stage of labor coincides with the longest diameter of the pelvis. The head enters the pelvis with the occiput (or back of the skull) being directed toward one ilium, and the face toward the other, while, at its final emergence, the face is turned toward the sacrum and coccyx.There can be no doubt that thescrew-like or rotatory motion which is thus given to the fetal head, renders its passage through the the pelvis more easy than it would otherwise have been.
There are well-marked differences, chiefly having reference to the act of parturition, between the male and female pelvis. In the female, the bones are lighter and more deli cate than in the male, and the muscular impressions and emineuces are less distinctly marked. The iliac fosste are large and expanded, and hence the great prominence of the hips. The several diameters (particularly the transverse diameter of the brim, which measures only 5.1 in. in the male) are somewhat greater; and the pubic arch is wider by about 10 degrees; the sacrum also is wider and less curved.
It is worthy of notice that the pelvis of the negro is smaller in all its dimensions than that of the European, and presents a partial approximation to that of the monkey, espe cially in the deficiency of its width. This difference is very much more obvious in the male than in the female negro; and parturition in the black races is facilitated both by the sacrum being less curved, and by time foetal head being of smaller dimensions. In the apes and which approach most nearly to man; the pelvis is longer and narrower, and much less curved than in the human subject. In other mammals, the differences are for the most part the same in kind, but greater in degree. In many of (bats) and insectivora (as the mole), the pubic bones arc only loosely con nected by a small ligament, or there is a complete opening between the bones (as occurs normally- in birds), an arrangement by which the act of parturition in these animals is much facilitated. The pelvic bones are very simple in the cetacea, in some cases being represented by two simple elongated bones lying near the anus, and converging from opposite sides (a transverse connecting piece being sometimes but not always present); in others, by a small V-shaped bone, while sometimes (as in manatus) they seem to be entirely wanting. The additional pelvic bones in the non-placental mammals have been already noticed in the articles on the MAnsurfATA and IIIONOTRENUTA. In the echidna (belonging to the latter order), the acetabulum is perforated, as occurs normally in birds. In birds, in addition to the peculiarity just noticed, we find the pelvis open in front (or, more correctly, inferiorly), there being no union of the pubic bones in any bird except the ostrich. This normal incompleteness of the pelvic ring is obviously for the purpose of facilitating the passage of the eggs. It is unnecessary to trace the further degrada tion of the pelvic bones in the reptiles and fishes.