It's a shadow.
I would explain anatomically, but my vocabulary on that front consists mostly of 'thingy'.
EDIT: Tendon! Thank you, Google.
(
MrCanoehead Marzipaaaaaaaaaaaaaaaaaaaaan!,
Sun 28 Jul 2013, 12:28,
archived)
"thingy" is the best word ever
and is used extensively in the anatomical,surgical and medical fields of communication.
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Elvis impersonator,
Sun 28 Jul 2013, 12:37,
archived)
"Nurse, fetch the machine that goes "ping" and pass me the discombobulator for this thingy."
That was a brilliant episode of E.R.
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Jabberwoc misses D.R. and Quinch,
Sun 28 Jul 2013, 14:56,
archived)