1. A 50 -year-old electrician reports pain in his left upper extremity and tingling and numbness in his 4th and 5th digits of his left hand. Ther e is mild swelling of the left hand. The man reports most of his pain and numbness occurs when he is doing electrical work with his arms overhead. X-ray reveals the presence of a cervical rib. Which of the following structures is most likely being compressed?