al MUSCLES ON THE BACK OF THE FOREARM. 331 
muscles are concealed by the flexor tendons and the muscles of the thumb and 
little finger, and are crossed by the deep palmar arch and the deep branch of the 
war nerve. Between the two heads of the first dorsal muscle the radial artery 
enters the palm, and the perforating arteries pass between the heads of the other 
dorsal muscles. 
The interossei muscles of the hand in some cases have a disposition similar to 
that of the corresponding muscles of the foot. 
THE MUSCLES ON THE BACK OF THE FOREARM. 
The group of muscles occupying the outer side of the elbow and the back of the 
forearm and hand include the supinator muscles of the forearm and the extensors 
of the wrist and digits, They are divisible into a superficial and a deep layer. 
SUPERFICIAL MUSCLES. 
The superficial layer comprises seven muscles, viz. from without inwards, the 
brachio - radialis, the two radial extensors of the carpus, the extensor communis 
digitorum and extensor minimi digiti, the extensor carpi ulnaris, and the 
anconeus, 
The brachio-radialis arises from the upper two-thirds of the external supra- 
condyloid ridge of the humerus, and from the external intermuscular septum. 
Occupying the outer side of the hollow of the elbow, the muscle descends along the 
outer border of the forearm, and ends about the middle in a narrow flat tendon 
which is inserted into the ridge on the front of the base of the styloid process of the 
radius. 
The muscle is superficial in its whole length. Near its origin it is separated 
from the brachialis anticus by the musculo-spiral nerve. It forms the outer 
boundary of the hollow of the elbow, and conceals the radial extensors of the carpus, 
and, in the upper two-thirds of the forearm, the radial artery and nerve. 
The extensor carpi radialis longior (m. ext. carp. rad. longus) arises from 
the lower third of the external supracondyloid ridge of the humerus, from the 
external intermuscular septum, and from the common tendon of origin of succeeding 
muscles from the external condyle. It ends in a tendon in the lower half of the 
forearm, which passes beneath the posterior annular ligament, to be inserted into the 
back of the base of the second metacarpal bone. 
The upper part of the muscle is concealed by the brachio-radials, and the lower 
part is crossed by the extensors of the thumb. It covers the supinator brevis and 
extensor carpi radialis brevior above, and the back of the radius and the carpus 
below. 
The extensor carpi radialis brevior (m. ext. carp. rad. brevis) arises from the 
common tendon, from the external lateral ligament of the elbow, from the fascia 
over it, and from intermuscular septa on each side. It passes down the back of 
the forearm in close relation to the previous muscle, to be inserted by a tendon 
into the back of the base of the third metacarpal bone. A bursa is placed beneath 
the two radial extensor tendons close to their insertion. 
Partially concealed by the extensor carpi radialis longior and the extensor 
muscles of the thumb, and having the extensor communis digitorum on its inner 
side, the muscle covers the supinator brevis and the lower part of the radius. 
The extensor communis digitorum arises from the common tendon, from the 
fascia over it, and from intermuscular septa on each side. Extending down the 
back of the forearm, it ends above the wrist in four tendons, of which the outermost 
often has a separate fleshy belly. After passing beneath the posterior annular 
ligament in a compartment along with the extensor indicis, the tendons separate 
on the back of the hand, where the three innermost tendons are joined together by 
two obliquely-placed bands. One passes downwards and outwards, and connects 
together the third and second tendons; the other is a broader and shorter band, 
_ which passes also downwards and outwards, and joins the fourth to the third 
tendon, 
