On the dorsal aspect of the forearm, 5 cun distal to the olecranon, between the radius and the ulna.
🧭 How to find it (practical)
With the arm extended and pronated, locate the point 7 cun above the dorsal wrist crease, or 5 cun distal to the olecranon, between the ulna and the radius.
🔬 Anatomy
Extensor digitorum communis and extensor carpi ulnaris muscles. Innervated by the posterior antebrachial cutaneous nerve and the posterior interosseous nerve.
💉 Needling
Perpendicular insertion, 0.5–1.0 cun.
📜 Etymology
Si (四), four; Du (瀆), river. The Yangtze, Yellow, Huai He, and Ji Shui rivers were called Si Du in ancient times. The point lies between the radius and the ulna, as if it were a river.
