BONES AND MUSCLES OF ARM AND HAND. 481 
be otherwise inapplicable. It is only. among the higher 
orders of Quadrupeds, however, that it can possibly be exe- 
suted ; for in the lower, the two bones are united more or less 
completely into one, or are articulated in such a manner as to 
be incapable of rotation. 
640. The fore-arm is bent upon the arm, chiefly by muscles 
that lie upon the front of the latter ; of these the principal is 
the biceps or two-headed muscle (‘, fig. 227), which arises from 
the coracoid process of the scapula, and from the top of the 
glenoid cavity, and is inserted into the radius a little in front 
of the elbow, forming a great part of the fleshy mass in front 
of the arm (fig. 219). The arm is straightened again by a large 
muscle, the triceps or three-headed muscle, which arises from 
the back of the humerus and scapula, and passes down to be . 
inserted into a projection of the ulna behind the elbow-joint, 
forming the fleshy mass at the back of the arm. The muscles 
which rotate the fore-arm arise from the lower end of the 
humerus, or from one of its own bones, and pass obliquely 
across to the other. 
_ 641. The Hand is anatomically divided into three portions, 
—the carpus, metacarpus, and phalanges (fig. 223), The 
tarpus, Which is the portion nearest the wrist-joint, is com- 
posed of eight small short bones, which are firmly united to 
each other by ligaments, but yet have a certain degree of 
motion permitted them ; these are arranged in two rows, of 
which one has a rounded surface, and enters into the forma- 
tion of the wrist-joint ; whilst the other has a series of shal- 
low pits, to receive the rounded’ heads of the metacarpal 
ones. These last almost precisely resemble the bones of the 
rs, and in the skeleton might be mistaken for their first 
oints ; but with the exception of that of the thumb they are 
all united to each other by ligaments and muscles, so as to 
form the compact framework which gives support to the palm 
of the hand. The metacarpal bone of the thumb is much 
nore free in its movements ; and it is chiefly by an alteration 
its direction, that the thumb can be opposed to the fingers. 
e thumb and fingers are formed by a series of small bones 
thich are termed the phalanges; of these there are only two 
the thumb, whilst there are three in the fingers. They are 
mt on each other chiefly by the action of the muscles that 
cupy the front of the fore-arm ; and they are extended or 
II 
