326 ELEMENTARY PHYSIOLOGY less. 



bulum (Fig. 98) fits so closely to the head of the femur, 

 and the capsular ligament so completely closes its cavity 

 on all sides, that the pressure of the air must be reckoned 

 among the causes which prevent dislocation. This has 

 been proved experimentally by boring a hole through the 

 floor of the acetabulum, so as to admit air into its cavity, 

 when the thigh-bone at once falls as far as the round and 

 capsular ligaments will permit it to do, showing that it 

 was previously pushed close up by the pressure of the 

 external air. 



12. The Various Movements of the Body. — The 

 different kinds of movement which the levers thus con- 

 nected are capable of performing are called flexion and 

 extension ; abduction and adduction ; rotation 

 and circumduction. 



A limb is flexed, when it is bent ; extended, Avhen it is 

 straightened out. It is abducted, when it is drawn away 

 from the middle line ; adducted, when it is brought to the 

 middle line. It is rotated, when it is made to turn on its 

 own axis ; circumducted, when it is made to describe a 

 conical surface by rotation round an imaginary axis. 



No part of the body is capable of perfect rotation . like 

 a wheel, for the simple reason that such motion would 

 necessarily tear all the vessels, nerves, muscles, &c., which 

 unite it with other parts. 



Any two bones united by a joint may be moved one 

 upon another in, at fewest, two different directions. In 

 the case of a pure hinge-joint, these directions must be 

 opposite and in the same plane ; but, in all other joints, 

 the movements may be in several directions and in 

 various planes. 



In the case of a pure hinge-joint, the two practicable 

 movements — viz., flexion and extension — may be effected 

 by means of two muscles, one for each movement, and 

 running from one bone to the other, but on opposite sides 

 of the joint. When either of these muscles contracts, it 

 will pull its attached ends together, and bend or straighten, 



