184 OSSEOUS SYSTEM. 



Q. What indicates the disposition to luxation in an ar- 

 ticulation? 



/?. The number and freedom of its motions. 



Q. What articulations afford extension, opposition, cir- 

 cumduction, and rotation? 



ft. The humerus and scapula, the femur and ilium. 



Q. What advantages arise from the location of these 

 extensive motions at the upper part of the extremity? 



tfl. The joints are less exposed to accident and luxation; 

 the powers of motion being located in the superior articu- 

 lations, the whole limb has extensive motion, thereby 

 making up for the necessarily limited motion of the joints 

 below. 



Q. What form of articulating surfaces is necessary for 

 these extensive motions? 



./?. That the receiving surface should be concave, and 

 the moveable one convex. 



Q. What are the principal differences between the mo- 

 tions of the humeral and femoral articulations? 



/2. In the humeral there is much circumduction, but 

 little rotation; in the femoral there is much rotation, but 

 little circumduction. There is pronation and supination 

 for the hand to compensate for the want of rotation in the 

 humeral articulation; while the rotation of the femoral ar- 

 ticulation renders pronation and supination unnecessary, 

 in the lower extremity. 



Q. On what principle in the mechanical arrangements 

 of the articulations, does the facility of circumduction de- 

 pend? 



*ft. It depends on the " axis of the bone, and that of the 

 motion being the same," as is seen in the humeral articu- 



