MORPHOLOGY OF THE EXTENSOR MUSCLES. 3 
the extensor aspect of the forearm. The trunk formed by the union of the posterior 
interosseous and the “nerve to the anconeus” gives off the nerve of supply to the 
extensor longus digitorum, and is joined a little lower down by a communicating 
twig* from the median nerve. It then passes under cover of the extensor ossis 
metacarpi pollicis, supplies every part of the deep stratum, including the metacarpal 
heads of the short extensor, and gives cutaneous branches to the digits. 
_ According to the above account, the muscles of this region in Hatteria form an 
exceedingly symmetrical plan, and suggest a very primitive condition. The devia- 
tions from an almost ideal symmetry are slight, and include (1) the development 
from the éxt. brevis digitorum of an eat. ossis metacarpi pollicis: (2) the unequal 
extension upwards of the origins of the slips of the short extensor: (3) the abortion 
of the ulnar metacarpal head of the short extensor of the fifth digit: (4) the 
absence of the aponeurotic slip from the ext. carpi radials to the short extensor, 
which is present on the ulnar side. More noteworthy are the complete segmenta- 
tion of the triceps from the muscles on the extensor aspect of the forearm, and the 
unsymmetrical condition of those portions of the radial and ulnar sectors which are 
inserted into the forearm—the supinator mass being very much larger than the 
anconeus. 
The muscles in the two limbs are very similar to one another. In the superficial 
stratum of the hind limb the three sectors are distinctly segmented. The trbial sector 
(tibialis anticus) arises from the upper part of the anterior surface of the shaft of 
the tibia, and also from the outer border, and, to a certain extent, from the posterior . 
surface of the same bone. It ends in a tendon which is inserted into the tibial side 
of the base of the first metatarsal bone, and sends a slip (fg. 11, 1) to the base of 
the proximal phalanx of the hallux. The fibular sector (peroneus) arises from the 
middle three-fifths of the shaft of the fibula. It terminates in a short tendon which 
is inserted into the base of the fifth metatarsal bone, and gives off a very distinct 
slip (peroneus quinti digitt) which joins that division of the short extensor which is 
inserted into the little toe. The intermediate sector (extensor longus digitorum) 
arises by a rounded tendon from the fibular condyle of the femur. I+ forms a 
fleshy belly which terminates in two tendons. The first of these is inserted into 
the bases of the second and third metatarsal bones; the second into the base 
of the fourth metatarsal bone. The division of the extensor brevis, which is inserted- 
into the third toe, passes between the two tendons of the long extensor. 
The deep stratum consists of a proximal part and of a set of metatarsal 
heads. It shows a great similarity to the corresponding stratum in the fore limb. 
* This twig passes downwards and backwards between the radius and the ulna to reach the posterior 
interosseous nerve. In Varanus it forms the chief nerve on the back of the forearm. 
* Gtruer’s account (Phil. Trans. 1867) differs materially from mine. He appears to have overlooked 
the greater part of the deep stratum, merely describing the eat. ossis metacarpt pollicis, and not mention- 
ing the ext. brevis digitorum. He also describes the whole of the ulnar sector as inserted into the ulna. 
