478 BONES OF SHOULDER : SCAPULA AND CLAVICLE. 
firmer attachment to the muscles that arise from it (fig. 226, 3 ). 
The scapula is never deficient in animals that possess a superior 
extremity, though sometimes it is very narrow. The muscles 
attached to it are chiefly those which draw the arm upwards, 
and which turn it on its axis. In Man, their actions are 
very numerous and varied; but in animals that only use 
their extremities for giving motion to the body, the muscular 
apparatus is much simpler, and the scapula is narrower (fig. 
229, o). This is particularly the case in Birds (fig. 250, o), 
the raising of whose wings in flight is an action that requires 
very little power, though for their depression or pulling-down 
great muscular force is needed. 
636. The Clavicle is a rounded bone, attached at one ex¬ 
tremity to the acromion-process of the scapula, and at the 
other to the top of the sternum. Its principal use is to keep 
the shoulders separate; and we accordingly find it strongest 
in those animals, the actions of whose superior extremities 
tend to draw them together; whilst it is comparatively weak 
or altogether deficient in animals, the actions of whose limbs 
naturally tend to keep them asunder. In Birds, the violent 
drawing-down of whose wings in flight would tend to bring 
the shoulders together if they were not prevented, there is 
not only a strong clavicle, but usually a second bone having 
a similar function (§ 668). In the Horse and other animals, 
on the contrary, the bearing of whose weight on their fore¬ 
legs tends rather to separate the shoulders than to bring them 
together, the clavicle is deficient. 
637. The Scapula is connected with the central framework 
of the skeleton by various muscles (fig. 226), which pass 
towards it from the spinal column and ribs, and which serve 
alike to fix it, and to assist in sustaining the weight which it 
sometimes has to bear. In Man these are numerous, and their 
actions are various ; since the scapula is left very movable in 
him, that the actions of the arm may be more free. In 
Quadrupeds it is generally more fixed; and the trunk is slung 
from it, as it were, by a muscle (the serratus magnus, 9 ) of mode¬ 
rate thickness in Man, but in these animals of great strength, 
which passes from the scapula to be attached to the ribs. 
638. The superior or anterior member itself is divided into 
three principal portions,—the arm, fore-arm, and hand. The 
arm is supported by a single long and cylindrical bone, which 
