586 FUNCTION OF THE MUSCLES IN THE BODY. 



If the radius is raised to the position E R lf the factor is A . a E. It should, how- 

 ever, be noted here also that B R t < B R; hence the absolute muscular energy 

 must be less in the more flexed position, as every muscle is able to lift less weight 

 with increasing contraction. What is thus lost in energy is made up in elongation 

 of the arm of the lever. 



5. Some muscles have a double motor effect, which they usually exe- 

 cute combined; for example, the biceps muscle is a flexor and a supi- 

 nator of the forearm. If one of these movements is prevented by other 

 muscles, the muscle does not participate in the execution of the other 

 movement. 



Examples. If the forearm be strongly pronated and then flexed, the biceps 

 does not participate; or if the elbow be tensely extended, supination is effected by 

 the supinator brevis alone, not by the biceps. The muscles of mastication furnish 

 another example. The masseter raises the lower jaw and at the same time pulls 

 it forward. If the depressed jaw, however, be kept drawn strongly backward, the 

 masseter does not participate in the succeeding elevation of the jaw. The tem- 

 poral muscle raises the jaw, and at the same time draws it backward. If the de- 

 pressed jaw be raised when drawn forcibly forward, the temporal does not par- 

 ticipate in its elevation. The muscles of this group execute this partial movement 

 only on the strongest exertion, or when the position of the bones is specially in- 

 fluenced by other mechanical factors. The flexors of the leg also exhibit interesting, 

 analogous relations. 



A muscle connected with one joint as a rule causes in a neighboring joint a 

 movement opposite to that to which it gives rise in the joint over which it passes. 

 For example, the brachialis anticus causes, in addition to flexion at the elbow- 

 joint, also backward extension at the shoulder- joint. 



6. Diarticular or poly articular muscles are those that pass over two or 

 more joints in their course from origin to insertion. In these muscles the 

 tendons may deviate from a straight line in certain positions, for example 

 the extensors and flexors of the fingers and toes in flexion of the latter ; 

 or the direction remains constantly straight, for example the gastroc- 

 nemius. The muscles of this group exhibit also the following interest- 

 ing conditions: (a) The phenomenon of so-called active insufficiency. 

 If the origin and insertion of a muscle are too closely approximated as a 

 result of certain positions of the joints over which it passes, it may happen 

 that the muscle is compelled to contract to such a degree before its action 

 becomes effective that further active contraction is not possible beyond 

 the point at which its effect may first become manifest. For example, 

 when the knee is flexed at an acute angle, the gastrocnemius is no longer 

 able to accomplish plantar flexion of the foot ; the traction on the Achil- 

 les tendon is made by the soleus alone, (b) Passive insufficiency is 

 exhibited by the polyarticular muscles under the following conditions: 

 In certain positions of the joints a muscle may already be so stretched 

 and made tense as from this position to limit certain movements of 

 other muscles like a rigid restraining band. For example, the gastroc- 

 nemius is too short to permit complete dorsal flexion of the foot when the 

 knee is extended. The long flexors of the leg arising from the tuber- 

 osity of the ischium are too short to permit complete extension at the 

 knee-joint when the hip-joint is flexed at an acute angle. The extensor- 

 tendons of the fingers are too short to permit complete flexion of the 

 joints of the fingers when the wrist -joint is completely flexed. 



In the dependent upper extremity movement of the forearm at the 

 elbow-joint is attended with a change in the position of the upper arm. 

 The long head of the biceps tends to rotate the upper arm backward with 

 the elbow-joint in a position between extension and flexion at a right 



