Which long bone ossifies first?

The clavicle is the first (long) bone to ossify in the body, starting at about the fifth or sixth week of fetal life. It is followed shortly by the femur, which first exhibits ossification at around the seventh prenatal week. The clavicle is also one of the last to finish growing (generally during the 25^th year)!

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.

The Carpal Tunnel

The ‘carpal tunnel’ consists of ten structures (9 tendons for three muscles, and the median nerve) that pass over the carpal bones and under the flexor retinaculum. The relationship of the ten structures to each other is important for identification as well as the understanding of Carpal Tunnel Syndrome.

Picture from: http://www.aidmycts.com/_img/carpal-tendon.jpg.

The four tendons of the Flexor digitorum superficialis m. are located anterior to (on top of) the four tendons of Flexor digitorum profundus m., and are ‘stacked’ with the tendons to digits III and IV over lying the tendons to digits II and V (see below, click pic to enlarge).

Picture from: 2006 Elsevier Inc. Atlas of Human Anatomy 4^th Edition, Frank H. Netter MD, NetterAnatomy.com

As you can see from the pictures, the Median n. is located anteriorly within the carpal tunnel. When this nerve is compressed due to various issues (swelling of surrounding tissues, mechanical impingement, etc.), often resulting from repetitive motion, the result may be Carpal Tunnel Syndrome. A common treatment for this condition is the surgical release of the transverse carpal ligament (flexor retinaculum).