bircidni lujvo

c1=b1 is an elbow of body x2=c3=b2.

Cf. degycidni.

In notes:

birka
x1 is a/the arm [body-part] of x2; [metaphor: branch with strength].
degycidni
c1 is a knuckle on limb d2 of body c3=d3.