This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The thumb has two phalanges and the fingers each three. These are called the proximal, middle, and distal phalanges, also the first, second, and third phalanges. The thumb has only a proximal and a distal phalanx.
Into the middle phalanges on their palmar surfaces are inserted the flexor sublimis digitorum tendons and into the distal the flexor profundus (Fig. 362). There is only one long flexor to the thumb and it is inserted into the distal phalanx.
Fig. 364.- - Dorsal view of the extensor tendons of the finger.
(Fig. 363). On the dorsum of the proximal phalanx the tendon splits into three parts. The middle slip inserts into the bases of the middle phalanges, while the two lateral slips, after receiving the insertions of the lumbricales and part of the insertions of the interossei, insert into the bases of the distal phalanges of the fingers (Fig. 364). The thumb has two separate extensors, the extensor brevis pollicis and the extensor longus pollicis.
Into the bases of the proximal phalanges are inserted the remaining portion of the tendons of the interossei muscles, which move the fingers toward and from one another, and slips from the palmar fascia. The main function of the interossei and lumbrical muscles is to extend the distal and middle phalanges and to flex the proximal ones. When, therefore, most of them are paralyzed, as occurs when the ulnar nerve is divided, the distal and middle phalanges are flexed and the proximal phalanges extended, forming the claw-hand (main griffe) of Duchenne.