
I thought that sinew was like the cartilage you occasionally find in chicken nuggets, all slimy and rubber-like. But actually it's fibers that are found inside a long tendon found in big animals and once you take off the tough outer shell, the inside consists of thin fibers stuck together and they can be separated into sewing yarns.
Source:
https://neolithichouses.wordpress.com/2 ... eer-sinew/
http://stitchinguphistory.blogspot.fi/2 ... -back.html