The contrahens muscle layer in Tarsius.

@article{Day1975TheCM,
  title={The contrahens muscle layer in Tarsius.},
  author={Michael Hardy Day and Stephen Iliffe},
  journal={Folia primatologica; international journal of primatology},
  year={1975},
  volume={24 4},
  pages={
          241-9
        }
}
The dissection of five hands from four specimens of Tarsius sp. revealed an elaborate contrahens muscle group that may facilitate the specialised grip pattern in this form. Long muscle slips arise from the palm and pass to the distal phalanges of the digits. In addition, short muscle slips pass from one digit to the proximal phalanx of an adjacent digit. 
BETA