MORPHOLOGY OF THE EXTENSOR MUSCLES. 9 
others. The ulnar sector is divided, as usual, into the ext. carpi ulnaris and the 
anconeus. The former arises, as in Man, from the external condyle of the humerus, 
but does not receive any fibres from the ulna. Its tendon passes through the same 
compartment in the annular ligament as the tendon of the ext. minimi digiti, and 
then divides into two parts. The deeper part is joined by a tendinous slip from the 
abductor minimi digiti, and is then inserted into the base of the proximal phalanx 
of the little finger. The superficial part (represented in the figure as drawn out- 
wards by a pin) joins the other two extensor tendons of the little finger, and thus 
reaches the ungual phalanx of that digit. The normal insertion of the ext. carpi 
ulnaris has therefore disappeared, and the entire muscle is converted into an ulnaris 
quints digitt. The anconeus is very deeply placed. It arises in common with the 
ext. carpi ulnaris from the external condyle, and is inserted into the upper third of 
the shaft of the ulna. It is covered at its insertion by the ext. indicis et pollicis. 
Radial sector.—The ext. carpi radialis brevior is a large and well-developed muscle. 
It arises from the external condyle and from the supra-condyloid ridge, and is 
covered at its origin by the triceps. Its tendon passes through the posterior annular 
ligament in the usual situation, and divides into three strong slips which are 
inserted into the bases of the second, third, and fourth metacarpal bones. The eat. 
carpi radialis longior is about one-fifth the size of the preceding muscle, by which 
it is concealed for the greater part of its extent. It arises from the external supra- 
condyloid ridge of the humerus, anterior to the uppermost fibres of the ext. carpi 
radialis brevior. It is continued into a slender tendon, which is inserted into the - 
base of the first metacarpal bone, and also gives off a fibrous expansion which blends 
with the ligamentous structures on the back of the carpus. The swpinator brevis 
arises from the external condyle and from the ridge immediately above the condyle, 
in common with the ext. carpi radialis brevior. It is inserted into the upper third 
of the external border, and slightly into the anterior surface of the radius. The 
supinator longus is entirely wanting. 
Deep layer.—The ext. ossis metacarpi pollicis (Fig. 6, ab. p. 1.) arises from the 
upper third of the posterior surface of the shaft of the ulna, and is covered from 
above downwards by the anconeus, the ext. indicis et pollicis and the ext. communis 
digitorum. Its tendon crosses the radial extensors of the carpus in the usual way, 
and passes through the outermost compartment of the annular ligament to be 
inserted into the metacarpal bone of the thumb. It is joined, close to its insertion, 
by a remarkable tendinous structure (Fig. 6, 7). This tendinous slip arises from 
the ulnar side of the carpus, and lies under cover of all the extensor tendons, except 
the ext. carpi radialis longior. It may perhaps represent a remnant of the primi- 
tive carpal origin of the ext. ossis metacarpi pollicis. The eat. indicis et pollicis* arises 
from the upper half of the posterior surface of the shaft of the ulna, extending 
* This muscle probably represents the ext. secundi internodii pollicis, the ext. indicts, and the occa- 
sional ext. medi of Man. 
B 
