Each o.. innominattun articulates with its fel low of the opposite side through the intervention of the intooaS( (MS fibro-rarlilage, which unites the two surfaces of the pubic hones, with the sacrum, and with the femur (at the aeetabitlum). No less than thirty-five muscles are attached to this bone. some proceeding to the region of the back, others forming the walls of the abdomen, others forming the floor of the pel vis, others passing downward to the lower ex tremities. etc. 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 sub ject, it is sifflicient here to remark that, col lectively. they form a triangular bony mass (with the base upward, and with a concave anterior surface). which constitutes the posterior part of the pelvic ring. tee Fig. 2.
The pelvis, considered as a whole, is divisible into a false and true pelvis. The false pclris is all that expanded portion which is bounded later ally by the iliac bones, and lies above the promi-, nent line termed the linea ileo-pertinea (see Fig. 2) ; while the true pelvis is all that part of the genera] pelvic cavity which is situated below that line. The broad, shallow cavity of the false pelvis serves to support the weight of the intes tines: while the rectum. bladder. and part of the generative organs lie in the cavity of the true pelvis. The upper aperture of the true pelvis is termed the inlet. It is somewhat heart-shaped in form, and has three principal diameters—an antero-posterior (or sacro-puhie), which extends from the angle formed by the ;imam with the last lumbar vertebra to the symphysis pubis, or point of union of the two pubic bones; the trans rerse, at right angles to the former, and extend ing across the greatest width of the pelvis; and the oblique, extending from the sacroiliac sym physis (or union). on one side. to the margin of the brim corresponding with the acetabulum on the other, The diameters of the outlet are two— an antero-posterior, extending from the tip of the coccyx to the lower part of the symphysis pubis, and a tt•an.crrr.cr, from the posterior part of one isehiatic tuberosity to the same point on the op posite side. As the precise knowledge of the di ameter 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—antvro-po-terior, 1.4 inches; transverse, 5.4 inches; oblique, 4.3 inches. Di ameters of outlet—mitero-posterior, 5 inches; transverse, 4.3 inches. Depth of time true pc e is— posteriorly, 4.5 inches; in the middle, 3.5 inches; anteriorly, 1.5 inches.
The 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 flu to 65' with the horizon. According to (:ray, the extremity of the coccyx is in the female, when standing upright, about one-half inch higher than the lower edge of the symphysis pubis, the upper 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 de tached 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 sup port the viscera when the body is in an erect posture; but it is of much more importance in its relation to the act of parturition. If all the an
tero-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 has to fol low this curve. the difficulties of parturition are Much greater than if the axis of the pelvis had been straipitt, as in the other vertebrata. With out entering into unnecessary details, we may remark, generally, that the foetal 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 pel vis 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 the screw-like or rotatory motion which is thus given to the foetal head ren ders its passage through the pelvis more easy than it would otherwise have been.
There are well-marked differences. chiefly hay ing reference to the act of parturition. between the male and female pelvis. In the female the bones are lighter and more delicate than in the male. and the muscular impressions and emi nences are less distinctly marked. The iliac fos so• are large and expanded. and hence the great prominence of the hips. The several diameters (pirticularly the transverse diameter of the brim, which measures only 5.1 inches 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 approxi mation to that of the monkey, especially in the deficiency of its width. This difference is very much more obvious in the male than in the fe male negro; and parturition in the black races is facilitated both by the sacrum being less curved. and by the firth) head being of smaller dimensions. In the apes and monkeys. 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 differ ences are fur the most part the same in kind, but greater in degree. In many of the Cheiroptera t bats) and Insectivora (as the mole) the pubic bone; are only loosely connected by a small liga ment. or there is a compbqe opening between the bones (a; occurs normally in birds), an arrange ment by whielt the act of parturition in these animals is much facilitated. The pelvic are very simple in the Cetacea, in some cases being represented by two simple elongated bones lying near the antis, and OM erging from opposite sides (a transverse connecting piece being sometimes but nut always present 1 : in others. by a small V shaped bone; while las in Mallows) they seem to be fmtirely wanting. The additional pelvic bone; in the nomplaeental mammals have been already noticed in the article on the MAt: SUPI. LIA. In the echidna, which belongs to the Prototheria q.v.), the acctabulumni is perforated. as occurs normally in birds. In birds. in addi tion to the peculiarity just noticed. we find the pelvis open in front i or. more correctly. infe riorly)., there being no union of the pubic bones in any bird except the ostrich. This normal incompleteness I if the pelvic ring is obviously for the purpose of faeilitating the passage of the eggs.