Positional Changes and Rotation of the Limbs

Positional Changes and Rotation of the Limbs

Limbs:During their development, the limbs undergo several changes of position. In the very early development they point in a caudal direction, bat soon project outward (ie, laterally) almost at right angles to the body wall. At this stage the flexor surface of each limb* is ventral and the extensor surface is dorsal. The preaxial border of each limb* lies cranially, whereas the postaxial border lies caudally. Next, the limbs* bend in a ventral direction at the elbow and knee. Consequently, the elbow and knee now point outward, while the palms and soles face the trunk. Finally both the sets of limbs undergo a rota- tion 90 of degree on their longitudinal axes, but in opposite directions. The upper limbs* rotate laterally, so that elbows come to face posteriorly and the extensor muscles come to lie on the lateral and posterior aspects of the
limb*. The lower limbs rotate medially through 90 degrees, so that the knees
face ventrally and the extensor muscles come to lie on the anterior aspect
of each lower limb*.

Innervation of the Limbs

During the 5th week of development the ventral rami of certain spinal nerves unite to form plexuses for limbs*. The brachial plexus (C5-8, T1) for the upper limb*, and the lumbo sacral plexus (L4, 5, S1-3) for the lower https://en.wikipedia.org. The main (named) nerves develop from these plexuses and extend progressively into the developing limbs*. Due to mixing of nerve fibers in the plexuses, the main nerves of the limbs contain fibers from several spinal segments. (eg, C5-T1, in the radial nerve and C7-T1 in the ulnar nerve). The main nerves then give branches to the muscular groups and to the skin and by the end of the 7th week the pattern is similar to that in the adult.