Human Anatomy - Kotsan I. Y. 2009

Skeleton of the lower limb
Phylogenesis of the limbs

Two pairs of limbs are typical of almost all vertebrates. Like the entire Skeleton, vertebrate limbs have undergone a long phylogenetic evolution since the first vertebrates moved from Water to land and began an amphibious lifestyle. Ancient amphibians, such as the fossil stegocephalians, resembled their fish ancestors, but they developed new Organs of locomotion—limbs designed to support their body weight on land.

These earliest limbs originated from the paired fins of fish. They possessed five digits, a trait preserved across all major vertebrate groups up to humans. Only in some vertebrates has the pentadactyl limb been reduced in connection with their mode of life. For instance, birds have a reduced number of Carpal Bones and digits, especially in their forelimbs, which are modified into wings. In many mammals, locomotion shifted from supporting the body on the entire sole to walking solely on the Phalanges—digitigrady. Others rely exclusively on the terminal phalanges, which developed protective adaptations known as hooves—unguligrady. A horse serves as an example of a single-hoofed (odd-toed ungulate) animal. Artiodactyls (even-toed ungulates) developed two digits with two hooves, accompanied by a reduction in the number of digits.

In a typical pentadactyl limb of terrestrial vertebrates, two main sections are distinguished: the girdle and the free limb, which consists of three segments—the upper arm or thigh, the forearm or lower leg, and the hand or FOOT.

The Pectoral Girdle of the stegocephalian (a fossil amphibian ancestor) consisted of a posterior section—the scapula; a separate anterior bone, the coracoid, which in later evolution fused with the scapula to form the coracoid process; and a typical dermal bone, the clavicle, which is well-developed in higher mammals and especially in primates and humans in connection with the high mobility of the upper limb. The Pelvic Girdle of the stegocephalian contains the primordia of all three pelvic bones—the ilium, ischium, and pubis—though they are separated by a broad cartilaginous area that also houses the acetabulum for articulation with the free limb. Already in reptiles, all three parts ultimately fuse in the region of the acetabulum to form a single pelvic bone. In higher vertebrates, particularly in apes and humans, the two pelvic bones connect at their ventral ends, while the sacrum wedges between them dorsally, forming a ring-like Structure known as the pelvis. In animals, the pelvis serves as support for the hind limbs, whereas in humans, it Supports the lower limbs in adaptation to bipedal posture. In humans, the iliac bones expand significantly in the lateral directions, taking on the function of supporting the Internal Organs of the Abdominal cavity.

Humans are unique among all vertebrates in having adopted an upright posture, relying exclusively on their hind (lower) limbs for support. The human upper limbs, having become upper extremities due to the erect posture, were completely freed from locomotor Functions, enabling them to perform highly refined movements. Consequently, the BONES OF THE arm are lighter and more delicate in structure than those of the leg, and they are joined by mobile articulations. The freedom of movement of the human upper limb is further facilitated by the clavicle, which positions the free upper limb laterally. The human hand adapted specifically for labor: the carpal bones are small and flexibly articulated; the digits have elongated and become highly mobile; and the thumb is set at an almost right angle to the metacarpals, possessing high mobility and opposability to all other digits, thereby ensuring the grasping function of the hand during complex work activities.

The human lower limb serves to support the body, maintain an upright posture, and propel it through space. Accordingly, the bones of the lower limb are massive, and the joints between individual segments are less mobile than those in the upper limb. This functional divergence between the upper and lower limbs in humans is most clearly reflected in their distal segments—the hand and the foot.

The hand develops and refines as an organ of labor, whereas the foot functions as a body support, bearing its entire weight. The toes of the foot play no significant role in support and have thus become greatly shortened. The hallux (big toe) lies in the same plane as the other toes and lacks pronounced mobility.

The foot is a mechanically complex arched structure, functioning as a resilient support that helps cushion shocks and impacts during walking, running, and jumping.