MUSCULATURE. 273 
ridge. It becomes tendinous at a point slightly more distal than the longus, 
and the tendons of both muscles pass together beneath the tendon of the extensor 
brevis pollicis, and along a slight groove on the distal head of the radius. The 
tendon of the brevis division becomes very flat and inserts on the dorsal side of 
the third metacarpal at its proximal end. The condition of these two muscles 
is thus quite the same as in the Echidna. 
The extensor digitorum communis (Plate 1, fig. 2, ed) is likewise practically 
identical with that of the Echidna. It arises from two heads:— (1) a narrow 
muscular strip from the extreme anterolateral border of the external epicondylus 
and (2) a deeper muscle overlaid by the extensor digitorum lateralis and extensor 
carpi ulnaris. It arises along the proximal two thirds of the anterolateral 
border of the ulna medially to the line of contact with the radius from which 
also come a few fibers. Both heads unite at about two thirds the length of 
the fore arm and pass as a flat tendon along the depression between ulna and 
radius. On the carpus, the tendon divides into three branches, one each to 
the dorsal margin of the distal phalanx of digits 2, 3, and 4. In the Echidna, 
notwithstanding that the fifth digit is clawed and hence more functional as a 
digging organ than the clawless knob representing that digit in the Proechidna, 
the communis tendons are likewise three, one each to the same digits, although a 
fourth branch to digit 5 might have been expected. 
The extensor digitorum lateralis (Plate 1, fig. 2, el) is a very thin narrow 
band arising by a flat tendon from the lateral epicondylus of the humerus and 
from the under side of the extensor carpi ulnaris, for about 6 mm. out from its 
proximal end. It is thus largely covered by this muscle and its flat tendon 
appears just proximal to the wrist, and passing over the hand, inserts at the 
proximal end of the terminal phalanx of digit 4, underneath the lateral branch 
of the communis tendon. In the Echidna the condition is quite similar. 
The extensor carpi ulnaris (Plate 1, fig. 2, ew) is a flat muscle from a broad 
origin by tendinous fibers extending between the distal edge of the lateral epi- 
condylus of the humerus to the dorsal edge of the base of the ulna. The muscle 
tapers to a strong round tendon that passes over the distal head of the ulna 
where it runs along a shallow groove to the outer edge of the hand and inserts 
as in the Echidna into the first phalanx of the fifth digit at the proximal end. 
The supinator is a stout muscle, triangular in cross section, lying under 
cover of the extensores carpi radialis et ulnaris. It is inserted along the anterior 
surface of the proximal half of the radius. 
The extensor brevis pollicis (Plate 1, fig. 2, ep) is essentially as described 
