42 ANATOMY AND PHYSIOLOGY. 



blade is triangular, with one angle directed downward, one upward, 

 and one outward, and it covers the ribs from the second to the 

 seventh. It is not, however, attached to the ribs, but is separated 

 from them by a cushion of muscle on which it glides. At its ex- 

 ternal angle there is a' socket for the bone of the arm so shallow 

 that this bone is not laid into it, but merely against it, which is one 

 reason of the frequency of dislocations of the shoulder-joint. 

 The bone of the arm {humerus) is single, attached above to the 

 shoulder-blade, and below to the bones of the forearm, radius and 

 ulna, and forms at the elbow a perfect hinge-joint. It has a large 

 round head, which is united by a ball and socket-joint with the 

 shoulder, capable of motion in every direction, and by a hinge- 

 joint with the forearm, capable of flexion and extension. It has 

 two projections externally and internally just above the elbow, 

 which give the breadth to this part of the limb, and to which the 

 muscles of the forearm are attached. The bones of the forearm 

 are two, the radius and ulna, the former being on the outer side, 

 and the latter on the inner. The ulna is connected chiefly with 

 the elbow-joint, and the radius chiefly with the wrist ; so that when 

 a fall is received on the hand, the force is transmitted through the 

 radius much more than through the ulna, and hence the radius is 

 broken much more frequently than any other bone in the body. 

 The ulna is articulated very firmly to the humerus or arm bone, and 

 moves on it in flexion and extension ; it can be bent up very close 

 to it, and can be extended very nearly in a straight line with it. 

 The radius is very slightly connected with the arm bone, and has 

 a round head received into a cavity in the outside of the ulna, 

 while at its lower end it has a cavity in its inner side, which rolls 

 round the small lower end of the ulna. The effect of this arrange- 

 ment is, that the ulna has always the same face directed forward, 

 while the radius can roll round the ulna, so that its edge, or even its 

 back, can be turned forward, earring the hand along with it. This 

 motion is commonly said to take place in the wrist, but in reality 

 the wrist has nothing to do with it. It is called pronation and 

 supination ; the hand is said to be prone when its back, and supine 

 when its palm is turned forward. It is in this motion that the 

 greatest difference is observed between the forearm and the leg ; 

 had any such motion been permitted in the leg, it would have pro- 

 duced instability. The two bones are connected in their whole 

 length by a strong membrane, which gives origin to muscles, while 

 it does not interfere with the rolling motion. The two extremities 



