480 BONES AND MUSCLES OF UPPER EXTREMITY. 
muscles which move it are for the most part attached to its upper 
third ; and the chief of them are the pectoralis major (fig. 227, 
2, 8) which rises from the sternum and cartilages of the ribs, 
and consequently draws the arm forwards, inwards, and down- 
wards,—the latissimus dorsi (fig. 226, °), which rises from the 
spinal column and hinder part of the ribs, and consequently 
draws the arm backwards, inwards, and downwards,—and the 
deltoid (fig. 226, °), which arises from the upper edge of the 
clavicle, and from the ridge of the scapula, and is the chief 
muscle concerned in raising the arm, The first of these forms 
the principal part of the fleshy mass upon the front of the 
chest, and is the muscle which is so remarkably developed in 
Birds. It forms also the front boundary of the awillary space, 
or hollow of the arm-pit, the hinder boundary of which is 
formed by the second muscle. This space, of which we can 
distinctly feel the front and back walls when we raise the 
arm a little from the side, contains the large vessels and 
nerves proceeding to the arm, and also a number of lymphatic 
glands (§ 219). The deltoid muscle forms the thick fleshy 
mass on the top of the shoulder and on the upper part of the 
outside of the arm. 
639. In the fore-arm of Man there are two long bones, 
termed the Radius and the Ulna, which lie nearly parallel to 
each other ; the radius being on the outer or thumb side of the 
fore-arm, and the ulna on the inner. They are connected 
with one another, not only by ligaments at their extremities, 
but by a strong fibrous membrane that passes between their 
adjacent edges, along their entire length. Nevertheless they 
have considerable freedom of motion, not only upon the 
humerus, but upon each other; so as to give to the fore-arm 
the power of rotation on its own axis, by which either the 
palm or the back of the hand may be turned upwards, The 
ulna is connected with the humerus, at the elbow, by means 
of a hinge-joint, into which the radius does not enter ; but it 
is the radius with which the hand is connected at the wrist, 
by a kind of ball-and-socket joint, the ulna having no direct 
share in this articulation: hence, while both bones move 
together in bending or straightening the elbow, we can make 
the radius roll round the ulna, carrying the hand with it. 
This movement is one of very great importance, in rendering 
the hand capable of a great variety of uses to which it would 
