The upper and the lower limbs, both are built on the same basic principle. Each limb is made up of two portions : proximal and distal.
The proximal part is also called limb girdle that attaches the limb to the trunk. The distal part is free and consists of proximal, middle, and distal segments; which are known as arm, forearm, and hand respectively in the upper limb; and thigh, leg, and foot respectively in the lower limb.
Homologous Parts Of The Upper And Lower Limbs
|Upper limb||Lower limb|
|Shoulder/pectoral girdle||Hip girdle/pelvic girdle|
|Shoulder joint||Hip joint|
|Elbow joint||Knee joint|
|Wrist joint||Ankle joint|
|(a) Carpus||(a) Tarsus|
|(b) Metacarpus||(b) Metatarsus|
|(c) Fingers*||(c) Toes*|
A short account of the development of the limbs further makes it easier to understand the differences between the upper and lower limbs.
The development of upper and lower limbs begins in the 4th week of intrauterine life (IUL). A pair of small elevations appears on the ventrolateral aspect of the embryo called limb buds. The anterior pair of the upper limb buds appears opposite the lower cervical segments. The posterior pair of lower limb buds appears 3 or 4 days later at the level of lumbar and upper sacral segments. Thus during an early stage of development all the four limbs appear as paired limb buds. First they are simple flipper-like appendages so that the upper and lower limbs are similar in their appearance. Each has dorsal and ventral surfaces, and preaxial and postaxial borders. The preaxial border faces towards the head. Later in the development, the ends of limb buds become expanded and flattened to form the hand and foot plates in which the digits develop. The digits nearest to the preaxial border are thumb and big toe in the upper and lower limbs, respectively. The limbs then rotate.
The lower limb buds rotate medially through 90° so that their preaxial border faces medially and their extensor surface faces forwards. The upper limb buds on the other hand rotate laterally through 90° so that their preaxial border faces laterally their extensor surface faces backwards.
|Upper limb||Lower limb|
|Function||Prehension (i.e., manipulation of objects by grasping)||Locomotion and transmission of weight|
|Bones||Smaller and weaker||Larger and stronger|
|Joints||Smaller and less stable||Larger and more stable|
|Girdle||Pectoral girdle||Pelvic girdle
|Preaxial border||Faces laterally||Faces medially|