<prompt> Structure lying at marker C ?Subclan veinSubclan aeryScalenus anteriorScalenus medius  <response>: Scalenus anterior muscle Attaches to the scalene tubercle on the innermedial border of 1st rib. Its tendon is sandwiched between subclan vein passing anterior and subclan aery passing posterior. Both vessels cross over the superior surface of 1st rib. Key: A - Area for attachment of scalenus medius B - Groove for subclan aeryC - Scalene tubercle attachment of anterior scalene D - Groove for subclan vein 