Extra cervical rib usually compresses which pa of brachial plexus -

THORACIC OUTLET SYNDROME Neurological and vascular symptoms and signs in the upper limbs may be produced by compression of the LOWER TRUNK of the brachial plexus (C8 and T1) and subclan vessels between the clavicle and the first rib. The subclan aery and lower brachial trunk pass through a triangle based on the first rib and bordered by scalenus anterior and medius. These neurovascular structures are made taut when the shoulders are braced back and the arms held tightly to the sides; an extra rib (or its fibrous equivalent extending from a large costal process), or an anomalous scalene muscle, exaggerates this effect by forcing the vessel and nerve upwards. REF:Apley&;s system of ohopaedics- 9th edn - pg no 292.