Jugular vein, omohyoideus
The sternocleidomastoid is crossed over in its middle part by the exterior jugular vein. Under certain conditions such as specific positions, emotion or diseases, the exterior jugular vein may present as very prominent.
Note : The external jugular vein enters the thorax ventral to the omohyoid.
Note : To differentiate in-between a vein and other structures such a as a tendon or a border of a muscle, is best done a tipping onto it with a finger. Veins collapse and refill, muscles and tendons remain firm.