Flexor digitorum superficialis and profundus mm.

The tendons of the Flexor digitorum superficialis (FDS) m. insert onto the intermediate phalanges of digits II-V, while the Flexor digitorum profundus (FDP) m. inserts onto the distal phalanges of those digits. This breaks the ‘rule’ in which more ‘superficial’ muscles insert farther distally along a limb than muscles originating from deeper levels. How is this possible?

It is possible because the FDS tendons split (see pics, below), allowing the tendons of FDP to come up from underneath and between the FDS tendon(s). Due to this, the FDS tendons can insert onto the intermediate phalanges, while the FDP tendons to insert onto the distal phalanges.

A good way to demonstrate this when presenting a hand dissection is to insert a probe under the FDP tendon after it comes up through the split FDS tendon, then pull up (anteriorly) to exacerbate the situation:

Pictures from http://www.turntillburn.ch/ttbs/en/med/ringband.htm.

These muscles have analogs in the feet: the Flexor digitorum brevis (FDB) m. works as the FDS does (although it is an intrinsic muscle of the foot), while the Flexor digitorum longus (FDL) m. (extrinsic to the foot) fulfills the same function as the FDP. The tendons are oriented as they are in the digits of the hand, with the tendons of the FDB splitting to allow the FDL tendons to pass through and insert onto the distal phalanges of digits II-V.