A single left sided Cervical rib, these can also occur on both sides.
There are many anomalies in the body and they are certainly not lying when they say that “everyone is different”. One anomaly which i find really interesting is the Cervical Rib, see if you have one (or two) yourself, 1 in 500 (0.2% of the population) of us will. Feel along the region between the Upper Trapezius (muscle between your neck and the top of your shoulder) and your Collar bone (also known as the Clavicle), you should be able to get your finger to dip down nicely into the gap between the two. If however, you cannot do this on one side, or occasionally even on both sides, as well as it feeling extremely hard (like bone) it may be due to an extra rib, also known as a ‘cervical rib’.
Rarely you can get symptoms down the arm as a result of the cervical rib, problems such as cold hands due to decreased blood flow, or/and tingling and numbness in the arm. These symptoms would be exacerbated by certain head positions as the rib clamps onto various structures. Chiropractors are well trained with taking and reading X-rays, cervical ribs are best seen with an X-ray or even better C-T scan (as you can see it in 3D, see above). They can occur on one side or both sides and are what are known as a “normal variant”.
See if you or someone you know has one or two! They are quite rare at 1 in 500 but are just another one of the cool normal variants that make us all so fantastically unique.