vni MfSt I.I. -MOVI.MI \ IS 73 



joint, a small diminution in length of the muscle will produce 

 a large movement of the hand carried at the end of the radius. 

 In forcible flexion of the arm, it is necessary for the scapula to 

 be fixed, and this is done by the simultaneous contraction of 

 muscles passing to it chiefly from the spine. 



Straightening or Extension of the Forearm. The 

 bent arm is straightened by the action of the triceps muscle, 

 situated on the back of the arm. The triceps muscle arises 

 partly by a tendon attached to the scapula, and partly by 

 two other portions attached to the posterior surface of the 

 humerus. Just above the elbow joint the muscle narrows to 

 a tendon which is inserted into the extreme upper end of 

 the ulna, the process of the bone which is distinctly felt at 

 the elbow. The ulna articulates with the humerus more than 

 one inch below this. When the muscle contracts it draws the 

 upper end of the ulna upwards, and so straightens the arm. 

 The arrangement is that of a lever of the first class. The 

 weight is at the hand at the one end of the ulna, the power 

 is applied by the muscle to the other end of the ulna, and the 

 fulcrum is at the elbow joint between the two, but close to 

 the insertion of the muscle. 



Flexion of the Leg at the Knee Joint. The leg is 

 bent by the action of the flexor muscles situated on the back of 

 the thigh, the chief of these being called the biceps of the leg. 

 Its origin is double, arising as it docs from the hip bone by 

 a tendon, and from the thigh bone by the direct attachment of 

 muscle fibres to the bone. The lower tendon passes over the 

 back of the knee joint, and is inserted into the upper end of 

 the fibula, at about two inches below the joint. The arrange- 

 ment is that of a lever of the third class, the fulcrum being at 

 the knee joint, the weight being the leg and foot, and the power 

 being applied just below the fulcrum. 



Extension of the Leg at the Knee Joint. The leg is 

 straightened, as in kicking, by the four extensor muscles lying 

 on the front of the thigh. The most important of these arises 

 by tendinous attachments from the hip bone above the socket 

 for the femur, passes over the hip joint, its belly lying on the front 

 of the thigh, and narrows below to a tendon which is inserted 

 into the patella, which is in its turn attached by a strong liga- 

 ment to the front of the tibia, about two inches below the 



