Internal structure of the innominate bone. — Like other irregular bones, the haunch bone is composed of cancellous structure, encased in layers of laminated bone, more or less thick, as strength and tenacity are re quired. The thickest and strongest part of the haunch-bone is the arched, rounded rib, ex tending from the cotyloid cavity to the au ricular surface and tuberosity of the ilium, which is the direct line of pressure from the spine to the legs. A cross section of this part of the bone is nearly an inch in diameter. generally, at its narrowest. point. The de scending rami of the ischia are the next in point of strength, for supporting the body in the sitting posture. These parts are composed chiefly of hard, laminated bone of dense section. The crest of the ilium is of a spongy structure, as also are .the thick masses of bone at the tuberosities of the ilium and ischium, and around the cotyloid cavity. The thinnest parts of the bone are at the bottom of this cavity, and in the centre of the iliac fossa, at which places is so little cancellous structure, that the thin laminx, forming the whole exterior of the bone, meet each other, and are so thin as to be sometimes translucent.
The SACRUM (Syn. TO tepov, Gr.; Os sa (Tani, basilare, Lat. ; Os sacrum, Fr. ; das heilige, or Kreuz Bein, Germ.) is a single pyramidal bone, flattened .antero-posteriorly, and placed in the median line, with the apex downwards, the anterior aspect being directed downwards and forwards, and the posterior upwards and backwards.
Its office is to form a basis of support to the spinal column and its appendages, and, acting like the keystone of an arch between the haunch-bones, to transmit to them its superincumbent weight. It also encloses and protects the terminal divisions of the spinal cord, and transmits them to the inferior parts of the body, and it completes the pelvic ca vity behind, and affords attachments to the strong muscles of the back. It presents for examination four surfaces ; anterior, posterior, and lateral, a base and an apex.
The base(fig.78.A) is directed forwards and upwards, and has a general resemblance to the upper surface of a vertebra, the lateral parts exactly corresponding to each other, as in most single bones. In the centre, anteriorly, is a large, plane, oval, articular surface (1) for fibro-cartilaginous union with the last lumbar vertebra, of which the long diameter is placed transversely, and about two inches long. On each side of this surface extends a large, fan shaped, flattish mass of bone (2), spreading externally, with a rounded anterior border, arching laterally, and forming part of the brim of the true pelvis. On the posterior border is seen a rounded prominence.. At the junc tion of the lateral masses with the oval arti cular surface posteriorly, two articular pro cesses (a,d) project upwards and backwards to articulate with those of the last lunabar ver tebra. Their roots are marked anteriorly by
deep grooves, in which lay the last lumbar nerves. Their articular facets are slightly con cave, circular, 'vertically placed, and directed backwards and inwards, and receive between them their counterparts on the last lumbar ver tebra. Springing from the roots of these pro cesses, two long lamina (1 n) pass backward and inwards with much downward inclination to meet each other in a stunted median spinous process (2 n), the commencement of the sa cral crest. They- correspond to the laminw and spines of the true vertebrw. The foramen (3n) enclosed by them is triangular in shape, and of considerable size, and is the superior opening of the sacral canal.
The apex (3 A) is a small oval, horizontal articular surface placed in the median line, with its long diameter half an inch in extent, and directed transversely. To it is articulated the base of the coccyx.
The anterior or pelvic surface is smooth and directed forwards and downwards, form ing the posterior wall of the true pelvis. It is vvidest above, opposite the lateral masses of the base (e). A little below this point it is about three-fourths of an inch nar rower (b). It then widens again to the extent of nearly half an inch (f ), and then gradually tapers to the apex. It is consider ably arched from side to side, especially at its superior part, where it has a transverse cur vature, varying from half to three quarters of an inch in central altitude. Longitudinally, also, this surface is curved to a still greater degree, and with greater variations, upon the comparative extent of which, in male and female, anatomists are much disagreed.
On each side of the median line are four holes, the anterior sacral foramina, separated from each other by three rounded transverse processes of bone about half an inch wide (4), and placed at equal distances of rather more than half an inch from the median line. The two upper holes are of equal size, and much larger than the two lower. Each is con nected to its fellow on the opposite side by four raised transverse lines (5), which mark the fcetal separation of this bone into five ver tebrx ; and extending outwards and down wards from each hole is a groove continued obliquely downwards to the borders of the bone (6). Below the last sacral hole, on each side, is a shallow notch, in the outline of the bone (c, ci), which is transformed into a foramen by the attachment of the upper transverse tubercles of the coccyx. There are many openings for nutritive arteries in this surface, directed generally towards the centre of the bone.