Anatomy of the arm and forearm

Segments of the upper extremity and anatomical position

The free upper extremity (membrum superior or extremitas superior) is topographically divided into three consecutive segments. The proximal segment is the arm (brachium), followed by the middle part — the forearm (antebrachium), and the extremity ends with the distal segment — the hand (manus).

A crucial rule to remember: the anatomical names of the extremity parts are not identical to the bone names. Any segment (e.g., the arm) is a complex structure. It includes not only the bony framework, but also the covering skin, muscle mass, and the blood vessels and nerve trunks passing through it.

The bony foundation is distributed as follows:

• In the region of the arm, there is a single humerus (humerus).
• The skeleton of the forearm is formed by two bones: the radius (radius) and the ulna (ulna).
• The skeleton of the hand has a complex structure and includes the carpal bones, metacarpal bones, and phalanges of the fingers.

When studying anatomy, all structures are viewed in the correct anatomical position. For the upper extremity, this means that the arm is lowered, the palm faces strictly forward, and the thumb points outward (laterally).

Structure of long tubular bones

The bones of the arm and forearm belong to the category of long tubular bones. Their main biomechanical function is the formation of reliable bone levers that ensure a wide range of movements of the free extremity segments. A characteristic morphological feature of such bones is the presence of a medullary cavity inside.

Anatomically, a long tubular bone is divided into several specialized zones:

In addition to tubular bones, there are also long spongy bones (such as the sternum and ribs), in the structure of which spongy bone tissue predominates, but they are not found in the free extremities.

Diversity of skeletal bone shapes

In addition to the long bones forming the basis of the arm and forearm, anatomy distinguishes other types of bones according to their shape and function.

Short bones are divided into tubular and sesamoid. Short tubular bones are localized in the distal segments (hand and foot), where they also act as levers, but now to ensure fine and precise movements. Sesamoid bones are unique in that they develop directly within the thickness of muscle tendons and perform the function of a biomechanical pulley. In the human body, there are two mandatory sesamoid bones: the patella (embedded in the tendon of the quadriceps femoris muscle) and the pisiform bone (located in the tendon of the extensor carpi ulnaris). Additional sesamoid bones belong to variant anatomy, are often found in the foot, and can increase the total number of bones in the human body (above the standard 206).

Flat bones perform protective and supportive functions. The bones of the limb girdles (scapula, hip bone) have a vast surface area for the fixation of powerful muscles. They service the most mobile joints of the human body — the shoulder and hip joints. The flat bones of the cranial vault have a special high-strength structure to protect the brain.

There are also irregular bones (combining different characteristics, typical of the skull base) and pneumatic bones. The latter are located in the skull around the nasal cavity; they are filled with air, which helps reduce the mass of the skull and creates natural acoustic resonators.

Class and exam questions

What else to study on this topic

• Bones of the hand: carpus, metacarpus, and phalanges
• Variant anatomy of accessory sesamoid bones of the foot
• Long spongy bones: structure of the ribs and sternum
• Structural features of the flat bones of the cranial vault
• Pneumatic bones of the skull and their role as resonators

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.