PELVIS. (treXtfE, Gr.; Pelvis, Lat.; le Bassin, Fr.; ders Bechen, Gerin.)— The pel vis is the bony girdle which connects the spinal column with the bones of the lower or hinder extremities.
It derives its name from its supposed resemblance in the human subject to a basin. Its figure, however, varies greatly in different animals. The description which follows, refers to the human 2naie pelvis, which may be taken as a standard cf com parison. It is composed of three principal pieces, two of which are symmetrical in shape, lateral in position, connected an teriorly, and called the innominate bones ; and the third, called the sacrum, intervenes between the former at their posterior extre mities, and connects them to the spinal column, of which it forms the inferior or posterior portion. Appended to the lower extremity of the sacrum is a small bone, the coccyx,— the representative of the caudal bones in the lower animals, — which, as influencing the shape and completing the formation of the walls of the pelvis, is considered as a part of it.
The INNOMINATE BONE (Os innominatum, coxarum, sive pelvis lateralis, Lat ; POs d'ile, coxaux, Fr.; das ungenannte Bein, Germ.; — figs. 76, 77.) is a bone of so irregular a shape as to leave it without a name in the fan ciful nomenclature of the older anatomists. It is broad and expanded at the upper ex tremity, rounded and perforated at the lower, constricted in the middle like a figure of 8, and bent into a curve, with its concavity di rected forwards, upwards, and inwards, at the upper end ; and backwards and inwards at the lower, so as to form a segment of the pelvic circle.
Its office is to support the bones of the lower extremities; to transmit to them from the sacrum the weight of the trunk in the erect position ; and to afford a basis of sup port in the sitting posture. It also forms a protecting enclosure to the important viscera within it, and gives attachment to the abdo minal and leg muscles.
This bone is usually described in three separate portions, into which it is separable in young persons, and which are called, re spectively, the haunch bone or ilium (das Hiip bein, Germ.; l'Ileon, Fr.); the seat bone or
ischium (ro taxtov, Gr. ; das Sitzbein, Germ. ; Ischeon, Fr.) ; and the share bone or pubis (6 Kretc, Gr. ; das Sekezambein or Schoossbein, Germ.).
Of these three, the ilium forms the upper expanded portion, and the pubes and ischium the lower perforated portion; the former being placed before, and the latter behind the opening. In the perfect bone, however, these three are completely soldered together by bony union in the central constricted portion, here each con tributes to form a deep cavity, externally, for the reception of the head of the thigh bone. From this cavity the three portions radiate ; the ilium upwards, the ischium downwards, and the pubes forwards, each contributing to support the thigh bone by its central extremity. The innorninate bone may, however, be most briefly described as one bone, consisting of two surface's, external and internal ; bounded by four borders, superior, inferior, anterior, and posterior.
The superior border, formed entirely by the ilium, is the most regular and the most ex tended. It forms an arch, directed from be hind forwards and outwards, and curved laterally so as to present, on looking at it from above, the shape of an italic f ; the smaller concavity being posterior and directed outwards ; and the larger, anterior and di rected inwards, contributing to form the ge neral concavity of the internal surface. This border is thickened in a somewhat irregular manner, forming what is called the crest of the ilium (a, c, b), upon which may be traced an internal and an external 4), and a rough broad central line. These ridges are caused by the attachment of the abdominal muscles. The external lip is called, by some authors, the su perior curved line of the ilium. The crest is very much thickened and irregular at the pos terior extremity, where it terminates in a back ward projection, the posterior superior spinous process (b). it is also thickened into an out ward projection a little in front of the centre (c), and also in a less degree at the anterior extremity, where it projects forwards, forming the anterior superior spinous process (a).