88 PHYSIOLOGY. 



bones of the fore-arm, called radius and ulna.\ |This bone is 

 susceptible of all kinds of motion, elevation, depression, ad- 

 vancing, retreating, circumlocution, and rotation Its scap- 

 ular extremity is lodged in a strong membraneous bag, called 

 the capsular ligament, and when the arm is raised up, the 

 bone slides downward in the glenoid cavity, and thus distends 

 the lower part of the capsular ligament. In every motion of 

 the arm, except in carrying it backwards, the scapular 

 readily moves or follows it ; it is therefore during motions of 

 the latter kind, that dislocations of the joint are most apt to 

 occur. If, therefore, the scapula could always follow the 

 motions of the arm, it would rarely be forced out of its 

 socket, and then only by extreme violence. 



47. In the fore-arm we find two kinds of motion, one at 

 the elbow, backward and forward, and also a rotary motion, by 

 which the palm is turned upward or downward, as occasion 

 requires. These motions are called supination and pronation. 

 Flexion and extension of the arm are performed by means of 

 the ulna, which being articulated, with the os humeri, by a 

 hinge joint, carries the radius along with it in all its move- 

 ments. Now while the larger part of the ulna is above, the 

 larger part of the radius is below, so that while the former 

 presents a large surface for articulation at the elbow, the lat- 

 ter does the same at the wrist, and this inverse arrangement 

 also contributes to the uniform diameter of the fore-arm. 

 While the fore-arm is thus attached to the os humeri, the 

 radius is attached to the wrist ; so that when we turn the 

 palm of the hand, the radius rolls on the ulna carrying the 

 hand with it. Indeed so admirable is this contrivance, that 

 both motions may be performed at the same time, for while 

 we are bending the arm, we may also be rotating or turning 

 it upon its axis. To facilitate these motions, it will be 

 observed that near the elbow, a tubercle of the radius plays 

 into a socket of the ulna, whilst near the wrist, the radius 

 finds the socket, and the ulna the tubercle. 



