As was pointed out by Henry Morris ("The Anatomy of the Joints of Man," p. 115), the bony pelvis is composed of arches. The two main arches are the femorosacral and the ischiosacral. These are strengthened by subsidiary arches which join the extremities of the main arches so as to strengthen and fix them.

Fig. 491.   The femorosacral arch. The main arch passes upward from one hip joint to the other through the sacrum: the subsidiary arch passes downward from one hip joint to the other through the pubes.

Fig. 491. - The femorosacral arch. The main arch passes upward from one hip-joint to the other through the sacrum: the subsidiary arch passes downward from one hip-joint to the other through the pubes.

Fig. 492.   The ischiosacral arch. The main arch passes upward from one tuberosity of the ischium through the sacrum down to the opposite tuberosity; the subsidiary arch passes forward from one tuberosity of the ischium through the pubes and back to the opposite tuberosity.

Fig. 492. - The ischiosacral arch. The main arch passes upward from one tuberosity of the ischium through the sacrum down to the opposite tuberosity; the subsidiary arch passes forward from one tuberosity of the ischium through the pubes and back to the opposite tuberosity.

Femorosacral Arch

This arch extends from the acetabula on the sides to the sacrum in the middle, which is its keystone. The weight of the body is transmitted downward through the spine to the sacrum, and then through the two sides of the femorosacral arch to the heads of the femurs. For an arch to be effective its two extremities must be firmly anchored, so that they do not separate when pressure is made on it. In artificial arches, as used in bridges, this separation is guarded against by a rod running from one extremity to the other, forming a chord of the arc. In the pelvis this mechanism is impossible, because this " tie-rod" would infringe on the cavity of the pelvis, and it is to obviate this that a counter arch is introduced. This secondary arch is formed by the rami and bodies of the pubic bones, and passes anteriorly from one acetabulum to the other on the opposite side. It is much weaker than the primary arch (Fig. 491).

Ischio-Sacral Arch

In sitting, the pelvis, viewed laterally, is in much the same position as in standing, being in both almost vertical and beneath the spinal column. The thighs, however, are horizontal and the bulk of the weight is supported by the tuber ischii. From the keystone or sacrum the weight is transmitted through the ilium and body of the ischium to the tuberosities on each side. This primary arch is strengthened by the secondary arch formed on each side by the ramus of the ischium and the descending ramus and body of the pubis. Notice that this likewise is weaker than the primary arch (Fig. 492).