Anatomists restrict “arm” to shoulder-to-elbow only
Everyday speech calls the whole upper limb an arm. Strict anatomy reserves the word for the humerus segment between shoulder and elbow; beyond that lies forearm, then hand—and a radial-nerve groove that makes midshaft fractures treacherous.
In casual English the arm is the upper limb. Academically it means only the upper arm between the glenohumeral (shoulder) joint and the elbow; the segment from elbow to wrist is the forearm, and beyond the wrist is the hand. The shoulder girdle’s bones, ligaments, muscles, and the axilla still count as upper-limb territory. Latin brachium, root of many anatomical names, can mean either the upper arm alone or the upper limb more broadly.
Three long bones define the scaffold. The humerus meets the scapula at the shoulder and meets the radius and ulna at the elbow, a complex hinge. A fascial sheet—the lateral and medial intermuscular septa—splits soft tissue into anterior and posterior osteofascial compartments that merge with the humerus periosteum. Anteriorly sit biceps brachii, brachialis, and coracobrachialis, all supplied by the musculocutaneous nerve from C5–C7. Posteriorly the triceps brachii alone is powered by the radial nerve.
Nerves and vessels explain why trauma maps so neatly onto deficits. The radial nerve, continuing the posterior cord, runs with the deep artery of the arm in the humerus’s radial groove; a shaft fracture there can lesion or transect the nerve and seed hematoma. Median and ulnar nerves transit the arm without supplying its compartments, yet at the elbow the ulnar nerve’s path behind the medial epicondyle leaves it vulnerable to condylar fractures. The brachial artery, continuing the axillary distal to teres major, is the main arterial trunk, traveling with venae comitantes toward the cubital fossa before entering the forearm. The “arm,” properly sliced, is a short bone corridor packed with named highways.
Source: Arm
