PELVIS, THE (from the Latin pelvis, a basin), is a bony ting interposed between the spinal column and the lower extremities, so as to transmit the weight of the former to the latter. Before considering the pelvis as a whole, it will be expedient to consider the individual bones of which it is composed. These, in the adult, are four in number, viz., the two ossa innorninata which constitute its sides and front, and the sacrum and coccyx, which complete it behind. The Os innomina tum receives its name from its bearing no resemblance to any known body, and is a large irregular-shaped bone. In the young subject, it consists of three separate bones, which meet and form the deep, cup-shaped cavity (the ace tabulum), situated a little below the middle of the outside of the bone, and in which the head of the thighbone rests. Hence it is usual to describe this bone as consisting of the ilium, the ischium. and the pubes. The ilium is the superior, broad. and expanded portion which forms the prominence of the hip, and articu lates with the sacrum. This bone may be described as divisible into an external and an internal surface, a crest, and an anterior and posterior border. The -external surface (see Fig. I.) is convex in front, and concave behind; it is bounded above by the crest, below by the upper border of the acetabulum (see Fig. II.), and in front and behind by the anterior and posterior borders. It presents various curved lines and rough surfaces for the attachment of the glutai and other powerful muscles connect ing the pelvis and the lower extremities. The internal surface, which is smooth and concave, has the same boundaries as the external, except iufcriorly, where it terminates in a prominent line, termed the linen ilio-peetinea. The sur face of the crest is convex, roughened, and sufficiently broad to admit of the attachment of three planes of ma.,eles. The borders will be sufficiently understood by a reference to fig. I. The i$eltiiim is the inferior and est portion of the bone. It consists of a thick and solid portion, the body (whose inferior border is termed the tuberosity), and a thin ascending portion, the rainy& In the ordi nary sitting position, the whole weight of the body rests on the isehirim and by sitting on the hands, we can usually feel the part (the tuberosity, see Fig. I. 15) through which the
weight is transmitted. The pubes is that portion which runs horizontally itiward from the inner side of the acetabolum for about 2 in., and then descends obliquely outward for about the same length, thus making an acute angle with its original direction. The former part is called the body, and the latter the minus, of the pules. The ramus is con with the ram us of tlw iseldum. Between the ischium and the pubes is a large aperture, known as the thyroid or obtoyttorfrramen, which in the living body is closed by a membrane termed the obtarator ligament.. The object of this large foramen is prob. ably to give lightness to the parts, without materially diminishing their strength.
The development of the os innominatum affords an excellent example of the general principles lain down in the article OssmcA•os. There tire no less than eight centers of ossification for this bone: three primary—one for the ilium, one for the ischium. and one for the pubes—and five secondary ones for various processes, etc. The first center appears in the lower part of the ilium, at about the same period that the development of the vertebra commences, viz., at about the close of the second month of foetal life; the second in the body of the isehinm, just below the twetabulum, at about the third month; and the third in the body of the pubes, near the acetabulum, during the fourth or fifth month. At birth the crest of the ilium, the bottom of the acetabulinn, and the rand of the ischinm and pubes. arc still cartilaginous. At about the sixth or seveneh year. these rand become completely ossified; next, the ilium is united to the ischinin; anti the pubes is joined to the other two in the acctabulnm. The complete ossification of the bone, from the secondary centers in the crest of the ilium, the tuberosity of the ischium etc.. is not completed till about the 25th year.