The ischium and postacetabular ilium originally enclose the ischiadic notch or incisura ischiadica.
This primitive condition occurs only in the Odontornithes, Ratitae and Tinami; in all others this notch becomes converted into a foramen ischiadicum, through which pass the big stems of the ischiadic nerves and most of the bloodvessels of the hind-limb.
The Crural Plexus is divided into a crural, ischiadic and pubic portion.
The ischiadic portion consists generally of five or six nerves, which leave the pelvis as one thick system through the ilio-ischiadic foramen.
The last nerve which contributes to the ischiadic plexus leaves the spinal column in most birds either between the two primary sacral vertebrae, or just below the hindmost of them, and sends a branch to the pubic portion which is composed of post-ischiadic nerves, partly imbedded in the kidneys, and innervates the ventral muscles between the tail and pubis, together with those of the cloaca and copulatory organs.