302 DISEASE IN WILD MAMMALS AND BIRDS 
diameter is greater than the lateral; the reverse is true 
in man. The long straight-sided quadruped pelvis is 
retained because of the direction of the forces from the 
legs, which is as much or more upon the anterior or 
pubic arch as upon the sacroiliac junction. In the semi- 
upright position of the monkey the force is directed 
backward and do\^Tiward upon the sacrum, this aiming 
to widen the pelvis by forcing the ilia apart. The effect 
of the upward force from the femora is to throw the 
pelvis upward and anteriorly by directing the line of 
action through the acetabula more toward the ventral 
surface. The sacroiliac ligaments hold the ilia firmly, 
their alae being spread outward by the force from above. 
Elevation of the pubes shortens their symphysis and the 
true conjugate. By these changes the birth canal is 
shorter, entirely bony, with the upper inlet on the same 
plane, and promontory and symphysis are near enough 
the same level to be met at the same time by the engaging 
head. In these pelves the anteroposterior diameter is 
still long and superior rotation is not necessary. 
In Man the force exerted on the pelvis from above is 
greatest among all animals and is greater when he is in 
active motion. The force is directed from above to the 
sacroiliac joint, the iliac bodies and the acetabular 
region wliile from below the pressure is directly exerted 
upon the last named. The force from above rotates the 
sacrum downward at its upper end, the attached sacro- 
iliac ligaments at the same time pulling the alae inward 
and throwing outward the lower end, the acetabular 
part. This tends to widen the pelvic box and to reduce 
its anteroposterior diameter. But the force acting from 
the legs and the adductors of the thigh push the lower 
parts of the ilia and the ischia and pubes apart, thus 
counterbalancing the effort of the force from above. The 
combined forces tend to bend the iliac bodies, thereby 
producing the curved lateral margins of the superior 
