48 PHYSIOLOGY OF THE HUMAN BODY. 
movable bones, which are- worked by means of muscles, like so many 
levers, and of a bony framework to which the limbs proper are attached, 
and which is fixed to the trunk. 
(1) Tae Upper Extremities—tHe Arms.—The bone in which is the 
socket for the first bone of the arm is a triangular, flat bone, lying on 
Kg phe back of the dorsal ribs, and called the sca 1 (1 in fig. 43). It lies 
quite freely on the ribs, ih so tends to give freedom of motion to the 
arm ; but it 1s very fray supported by the clavicle? or collar-bone (2 in 
fig. 43), which is attached to a projection on the scapula. The socket in 
the scapula for the reception of the head of the arm-bone, is a shallow 
cup-like cavity, into which the round head of the bone fits; and in con- 
Sequence of the shallowness of this socket, the arm has perfect freedom to 
move in every direction. Yet this arrangement has its disadvantage, from 
the greater danger there is of the head “of the bone being dialudaee from 
its place, as not unfrequently happens, security being thus slightly series 
for freedom of motion. It will be seen that in the case of the bip-joint, on 
the other hand, through the greater depth of the socket, freedom of motion 
is sacrificed for security and strength. THe Arm itself is divided into 
three parts—the arm proper (from the shoulder to the elbow), the fore- 
arm (between the elbow and the wrist), and the hand. The bony part of — 
the arm proper is a single hollow bone, called the humerus? or shoulder 
(3 in fig. 43). The shoulder-joint has already been described; at the 
lower end the bone is flattish, to allow of its being jointed at the elbow 
to the two bones of the fore-arm—namely, the radius* and the ulna® 
(4 and 5 in fig. 43). The ulna is connected with the humerus by a common 
hinge-joint ; while the radius has a pivot-joint, with a cavity in the end 
to receive a rounded knob on the end of the humerus, so that it is 
“capable of a rotatory motion. 
To these two bones at the wrist is attached THE HAND, which consists 
_,¢, altogether of 27 bones. First come the 8 carpaJ® or wrist bones, which are 
arranged in two rows—3 to 10 in fig. 44, 1 and 2 being the ends of the 
radius and ulna. The upper surface of the first row is convex, and fits 
into a cup-like socket in the lower end of the radius. The first carpal 
bone, 3, of the first row supports, by means of 7 and 8 in the second 
row, the bones of the thumb and forefinger (I and II); 5 in the first 
row, in like manner, supports, through 10 in the second row, the bones 
of the little finger and of the one next to it; 4 and 9 in the first and 
second rows, support the bones of the middle finger. These 8 carpal 
i Latin scapula = spatula, diminutive of spatha, a spade. 
alte 
hex 
2 From Latin clavicula, diminutive of clavis, a key, so called from its resemblance to a 
Roman key. 
3 Latin, ‘ the shoulder.’ 4 Latin, ‘a spoke of a wheel.’ 
5 Latin, ‘ the elbow.’ 6 From low Latin carpus, the wrist. 
