98 U.S. NATIONAL MUSEUM BULLETIN 2 73 



carpi ulnaris and the extensor digiti minimi. A small and round 

 tendon is formed when the muscle enters the central compartment 

 of the extensor retinaculum, where it lies deep to the tendons of the 

 extensor digitorum and on the medial side of those of the extensor 

 polHcis et indicis longus. In the hand, the tendon is deep to the central 

 carpal lamina of the extensor digitorum with which it is ensheathed 

 within the same cover derived from the superficial layer of the deep 

 manual fascia. It finally reaches the ulnar side of metacarpale III 

 at the metacarpophalangeal joint to end on the deep aspect of the 

 extensor expansion. This tendon contributes a thin and fibrous band 

 to the dorsum of the first phalanx of the third digit, and in the left 

 side of one male (fig. 30) a broader but thin fibrous band is suppHed 

 to the dorsum of the first phalanx of digit IV. 



M. extensor poUicis et indicis longus (fig. 29) takes origin by fleshy 

 fibers from the upper half of the lateral border of the ulna, above 

 the origin of the extensor tertii. Its fibers come from as high up as 

 the junction %vith the lateral border of the olecranon. They pass 

 distally toward the axis of the limb covered by the extensor carpi 

 ulnaris and the extensor digiti minimi. Before entering the central 

 and main compartment of the extensor retinaculum a short tendon 

 is formed which rapidly spUts into two, one for the second digit and 

 the other for the first. In the carpus and metacarpus they are both 

 apposed to the deep aspect of the central carpal lamina of the extensor 

 digitorum with which they are contained in the subdivision of the 

 superficial layer of the deep manual fascia. The tendon for the second 

 digit provides three expansions (fig. 30); a tendinous one goes to the 

 ulnar side of the second metacarpophalangeal joint where it ends on 

 the deep aspect of the dorsal extensor expansion on that side; a second 

 weak and membranous band goes to the dorsum of the base of the 

 first corresponding phalanx, while the third is similar to the first and 

 joins the radial extensor expansion on its deep aspect. The tendon 

 for the first digit ends dorsally on the base of the last polUcal phalanx. 

 It gives off membranous expansions at each side of the first meta- 

 carpophalangeal joint. These pass ventrally along the capsular liga- 

 ments to which they are connected. Obliquely running fibrous fascicles 

 pass from one tendon of the muscle to the other over the carpus. 



In talking about the representatives of the extensor digitorum 

 profundus in Alouatta fusca, Sirena (1871) calls the extensor pollicis 

 et indicis longus, m. extensor pollicis longus, but his description 

 practically matches mine in origin and insertions. The same is the 

 case with the extensor digiti tertii proprius which he calls m. extensor 

 indicis proprius, borrowing from the terminology of human anatomy. 



Nerve supply; Both components of the extensor profundus are 

 innervated by branches of the dorsal interosseous nerve. 



