Final answer:
The spinal gray of CN XI sends fibers to the sternocleidomastoid and trapezius muscles, which are involved in the movement and stabilization of the head and neck.
Step-by-step explanation:
The spinal accessory nerve, or cranial nerve XI, is responsible for the motor innervation of two specific muscles in the neck: the sternocleidomastoid and the trapezius muscles. These muscles are integral for movements such as flexion, extension, and rotation of the head. The sternocleidomastoid muscle allows for head rotation and lateral flexion, while the trapezius muscle contributes to the movement and stabilization of the scapula and serves as an antagonist in neck extension and hyperextension.