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 poUicis, 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 fPlate 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 nlnaris. 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 margm 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 passmg 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, eu) is a flat muscle from a broad 

 origin by tendmous 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 mserted along the anterior 

 surface of the proximal half of the radius. 



The extensor brevis pollicis (Plate 1, fig. 2, ep) is essentially as described 



