2 H. ST. J. BROOKS ON THE 
thus, the first slip into the bases of the first and second, and the remaining slips 
into the second and third, third and fourth, and fourth and fifth respectively : 
aponeurotic slips are prolonged also to the five bellies of the short extensor. 
Taking origin from the middle and distal segments of the limb is a deep 
stratum, including two sets of muscles: (1) a sheet of fibres, arising partly from the 
ulna and partly from the carpus (Fig. 9, ab. p. 1. and ex. b.); and (2) a series of 
muscular slips, arising from the dorsum and sides of the metacarpal bones. The 
former, or proximal part of the short extensor, divides below into six slips. Of 
these, the first (ext. ossis metacarpi pollicis) arises from the lower half of the ulna, 
and is inserted into the radial side of the base of the first metacarpal. The second 
(ext. sec. internodi pollicis), arising below the preceding, passes to the terminal 
phalanx of the thumb. The third (ext. indicis) arises chiefly from the carpus, and 
is inserted into all the phalanges of the second digit. The fourth and fifth arise from 
the carpus, and supply the middle and ring fingers. The sixth (ext. minima digitr) 
arises from the ulnar carpal bone and from the distal end of the ulna, and is inserted © 
into the phalanges of the fifth digit. 
The metacarpal heads of the short extensor,” partly overlapped by the proximal 
part of the muscle, arise in pairs from the dorsum and sides of the metacarpal bones. 
The first pair (Fig. 9, 1) jos the tendon of the ext. sec. int. pollicis; the second, 
third, and fourth join correspondingly the ext. cndicis, medu, and annularis. In 
the fifth digit, the ulnar part of the metacarpal head is wanting, and the muscle is 
represented by a single fasciculus (Fig. 9, 5) joming the radial side of the tendon of 
the ext. minimi digitt. 
The musculo-spiral nerve takes its usual course through the fibres of the 
triceps at the back of the humerus, and having reached the outer side of the arm, it 
passes through a foramen in the humerus immediately above the external condyle ; 
it then bends backwards and pierces the supinator mass, and is continued as the 
posterior interosseous nerve. While passing behind the arm the musculo-spiral 
gives off a nerve of considerable size (Figs. 9 and 10, «) which passes through the 
fibres of the triceps to reach the interval between the external condyle and the 
olecranon. Here it becomes superficial for a short distance, and then passes under 
cover of the anconeus, which it supplies. It then supplies the extensor carpi 
ulnaris, gives a twig to the supinator mass, and finally becomes incorporated with 
the posterior interosseous nerve. This nerve obviously corresponds to the “ nerve 
to the anconeus” of Human Anatomy, but it is relatively much larger in Hatteria 
than in Man, and takes a considerable share in the innervation of the muscles on 
* These accessory heads are very generally confounded with interossei dorsales. Their innervation, 
however, among other important considerations, shows that they are quite distinct from ‘ interossei.” 
These accessory heads attain their maximum development in Lizards. I find them very similarly disposed 
in Varanus. Firerincer, Mivart, and Sanpers describe them as equally well-developed muscles (but 
under the name of interossez) in Iguana, Platydactylus,.and Liolepis. In the Alligator (Fig. 16) they are 
more feebly represented. 
