6 H. ST. J. BROOKS ON THE 
In the fore limb of Menobranchus the superficial stratum is segmented, as in 
Hatteria, into three sectors. The extensor communis digitorum longus (Fig. 2, 2.5.) 
arises from the external condyle of the humerus, and passes downwards to the 
carpus, overlapping the radial and ulnar sectors to a certain extent. At the wrist 
it divides into four slips. The jist slip is inserted into the bases of the index and 
middle metacarpal bones ; the second slip into the middle and ring; the third into 
the ring and little metacarpal bones. The fowrth slip ends in a tendon which is 
inserted into the phalanges of the little finger. This part is joined by a slip from 
the flexor carpi ulnaris, which passes from the palmar to the dorsal aspect. The 
radial sector arises from the external condyle and from the ridge immediately above. 
It is an unsegmented mass inserted into the whole length of the posterior surface 
and outer border of the radius, and sending a strong slip to the radial side of the 
carpus. The ulnar sector arises from the external condyle, and is inserted into 
almost the whole length of the inner border of the ulna, but does not reach the 
carpus. It represents the anconeus, and the extensor carpi ulnaris is therefore 
unrepresented. In Menopoma and Cryptobranchus this sector has a slight carpal 
insertion. 
Deep layer.—The extensor ossis metacarpi pollicis arises from the extreme lower 
end of the ulna and from the carpus. It is inserted into the radial side of the base 
of the index metacarpal (the thumb being wanting) and into the adjacent part of 
the carpus. Its direction is nearly transverse. The extensor brevis digitorum con- 
sists of three small slips, which arise from the distal row of carpal bones and from the 
dorsum of the index, middle and ring metacarpal bones. The jirst slip is closely 
associated at its origin with the extensor ossis metacarpi pollicis, and passes with the 
latter muscle between the radial and intermediate sectors. It ends in a tendon, 
which is inserted into the phalanges of the index digit. The second and third slips 
pass between the divisions of the intermediate sector, and are inserted into the 
middle and ring digits. 
The musculo-spiral nerve escapes from under cover of the triceps in the usual 
situation, and divides into a radial and a posterior interosseus branch. The radial 
runs downwards under the integument to the outer side of the hand. The posterior 
interosseus nerve plunges into the unsegmented radial sector in which it ends. The 
“nerve to the anconeus” (Fig. 2, n.a.) escapes through the fibres of the triceps close 
to the elbow joint, and passes between the external condyle and the olecranon. In 
_this situation it gives off a twig, which is distributed to the integument on the ulnar 
border of the hand. It then passes under cover of the anconeus, supplies that 
muscle and the extensor longus digitorum, and runs nearly vertically down the fore- 
arm. At the wrist, it gives off the nerves of supply to the extensor ossis metacarpi 
pollicis and extensor brevis digitorum, and ends in cutaneous branches to the digits. 
Tt is therefore the main nerve, both muscular and cutaneous, to the back of the 
forearm. 
