Neck anatomy help!

There is a tendon-like part of my neck that I am trying to identify.  The only way I can feel it is if I tilt my head all the way back and slightly to one side. One I stretch my neck into this position the thing I am trying to identify starts at the very tip of my jawbone (slightly in front of my ear) on the side of my neck and runs down my neck slightly towards the front.  It is thin and stiff when my head is in this position.  I don't know if it is a tendon, muscle, vein or artery or something else. It is very distinctive when I stretch my head like this but in a normal relaxed position I cannot feel this part of my neck.  The reason I am trying to identify it is because I have had 2 lumps on one side of my neck that run along this this thing. My doctors have ruled out lymph nodes and nothing shows on an MRI. My blood work is normal.  Just wondering what this part of my neck is?  Thank you

[rattyrandi]

sounds like the sternocleidomastoid muscle.  sorry if the spelling is off.

[shirl000]

Thanks for the reply.  I looked up the sternocleidomastoid muscle and it looks more like it comes from under the ear and it kind of wide.  This comes from right at the tip of the jawbone where it bends, right before the ear.  It is also very thin.  I can't tell what it is.