PELVIS (Lat., basin). A bony ring inter posed between the spinal column and the lower extremities, so as to transmit the weight of the former to the latter. Before considering the pel vis 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 innominata which constitute its sides and front. and the sacrum and coccyx. which complete it behind. The as innornina turn receives its name from its bearing no resemblance to any known body, and is a large irrcgular•shaped bone. In the young subject it consists of three separate bones, which meet and form the deep enp-shaped cavity (the acetabulum), situated a little below the mid dle of the outside of the bone. and in which the head of the thigh-hone rests. Hence it is usual to describe this hone as consisting of the ilium, the ischium, and the polo's. The ilium is the supe rior, broad, and expanded portion which forms the prominence of the hip. and articulates with the sacrum. This hone may be described as. divi sible into an external and an internal surface, a crest, and :111 anterior and posterior border. The external surface (see Fig. 1) is convex in front and concave behind: it is bounded above by the crest, below by the upper border of the acetabu lum, and in front and behind by the an terior and posterior borders. it presents vari ous curved lines and rough surfaces for the at tachment of the glutei and other powerful mus cles connecting the pelvis and the lower extremi ties. The internal surface, which is smooth and concave, has the same boundaries as the external, except inferiorly. where it terminates in a promi nent line, termed the Iincn The surface of the crest is convex. roughened, and sufficiently broad to admit of the attachment of three planes of muscles. The borders will be sufficiently understood by a reference to Fig. I. The isehium is the inferior and strongest portion of the bone. It consists of a thick and solid por tion, the body (whose inferior border• is termed the tuberosity). and a thin ascending portion, the ram us. in the ordinary sitting position the
whole weight of the body rests on the isehium; nail by sitting on the hands we can usually feel the part (the tuberosity, see Fig. 1 ) through whle1i the weight is transmitted. The pubes is that portion which runs horizontally inward from the inner side of the acetabulum for about two ia;•hes. 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 pubes. The i'amus is continuous with the of the ischium. Between the isehium and the pubes is a large aperture, known as the thy roid or obturator foramen, which in the living body is closed by a membrane termed the obtura tor membrane. The object of this large foramen is probably 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 laid down in the article Ossfinc_moN. There are no less than eight centres 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 centre 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 fretal life; the second in the body of the ischium. just below- the aceta bulum, 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 acetabulum, and the rand of the isehium and pubes are still cartilaginous. At about the sixth seventh year these rani become completely ossified; next, the ilium is united to the isehium; and lastly, the pubes is joined to the other two in the aceta bulum. The complete ossification of the bone, from the secondary centres in the crest of the ilium, the tuberosity I if the ischium, etc., is not completed till about the twenty-fifth year.