aponeurosis

/ˌæpən(j)ʊˈroʊsəs/

noun

A sheet of pearly white fibrous tissue that takes the place of a tendon in flat muscles having a wide area of attachment.

The superficial branch, which forms the superficial palmer arch, descends a short distance into the palm and curves toward the thumb between the palmer aponeurosis and flexor tendons to form an arch.