332 THE MUSCULAR SYSTEM. 
The tendons are inserted in the following manner :—On the finger each tendon 
spreads out so as to form a membranous expansion over the knuckle and on the 
back of the first phalanx. The border of the tendon is indefinite over the meta- 
carpo-phalangeal articulation, of which it forms the posterior ligament. On the 
back of the first phalanx the tendon receives laterally the insertions of the interosse1 
and lumbrical muscles. At the lower end of the first phalanx the tendon splits 
into ill-defined median and lateral slips, proceeding over the back of the first 
inter-phalangeal articulation, of which they form the posterior ligament. The 
median slip is inserted into the back of the base of the second phalanx, while the 
two lateral pieces become united to form a membranous tendon on the back of 
the second phalanx, which, after passing over the second interphalangeal articula- 
tion, is inserted into the base of the terminal phalanx. 
The extensor communis digitorum is superficial in its whole length, and lies 
between the radial extensors of the carpus externally and the extensor minimi 
digiti internally. It conceals the supinator brevis and other deep muscles of the 
forearm, as well as the posterior interosseous vessels and nerves. The extensors of the 
thumb become superficial along its outer border in the lower third of the forearm. 
The extensor minimi digiti (m. extensor digiti quinti proprius) has an origin 
similar to and closely connected with that of the preceding muscle, from the 
common tendon, the fascia over it, and lateral intermuscular septa. Passing down 
the back of the forearm as a narrow fleshy slip, between the extensor communis 
digitorum and the extensor carpi ulnaris, it ends in a tendon which occupies a 
groove between the radius and ulna in a special compartment of the posterior 
annular ligament; on the back of the hand the tendon, usually split in two parts, 
lies internal to the tendons of the extensor communis digitorum, and is finally 
inserted into the expansion of the extensor tendon on the dorsum of the first 
phalanx of the little finger. 
The extensor carpi ulnaris has a double origin: (1) from the common tendon 
from the external condyle, from the deep fascia over it, and from the intermuscular 
septa; and (2) through the medium of the deep fascia, from the posterior border of 
the ulna in its middle two-fourths. The muscle ends in a tendon in the lower third 
of the forearm, which occupies a groove on the back of the ulna in a special com- 
partment of the posterior annular ligament, and is inserted into the back of the 
base of the fifth metacarpal bone. 
The muscle is superficially placed between the extensor minimi digiti and the 
anconeus, external to the posterior border of the ulna. It conceals the supinator 
brevis and the posterior interosseous vessels and nerve above, and in the lower two- 
thirds of the forearm covers the posterior surface of the ulna. 
The anconeus is a small triangular muscle arising by a separate tendon from 
the back of the external condyle of the humerus. Spreading out over the ulna, it 
is inserted into a triangular surface on the outer side of the olecranon process and 
back of the ulna, as low down as the oblique line. 
It is covered by the thickened fascia of the forearm giving insertion to the 
triceps muscle. It conceals the back of the elbow joint and part of the origin 
of the supinator brevis muscle. 
The epitrochleo-anconeus is an occasional small muscle arising from the back of the internal 
condyle of the humerus, and inserted into the inner side of the olecranon process. It covers the 
ulnar nerve in its passage to the forearm. 
DEEP MUSCLES. 
The deep muscles of the back of the forearm comprise five muscles, of 
which one, the supinator radii brevis, extends between the ulna and radius; the 
others are extensors of the thumb and forefinger: the extensor ossis metacarpi 
pollicis, extensor brevis and extensor longus pollicis, and extensor indicis. 
The supinator radii brevis (1m. supinator) muscle has a complex origin: (1) 
from the external condyle of the humerus; (2) from the external lateral and orbi- 
cular ligaments of the elbow joint; (5) from the triangular surface on the ulna 
below the lesser sigmoid cavity ; and (4) from the fascia over it. From this origin 
i 
