346 THE HUMAN MOTOR 



as the nature of the joint pre- 

 vents it. Hence the effect of 

 the lateral component BC 

 (vide fig. 246) is practically nil, 

 in flexion and extension. It 

 will be seen in fig. 247 that 

 the lower extremity of the 

 biceps takes the form of a 

 loop which encircles the 

 radius, which carries the 

 hand. Hence the whole 

 osseus segment can rotate in 

 siipination. At the same time 

 the tendon lengthens slightly FlG ' 

 to permit of the above move- Cause of rotation of the forearm, 

 ment (vide functional adap- 

 tations, 86) . Generally the forearm can be moved up or down 

 without any rotation taking place. If, however, the hand is 

 supporting a heavy weight, the motions of flexion and rotation 

 may take place simultaneously, the latter causing an unrolling 

 of the ligament of the tendon. A simple analogy is given by 

 R. du Bois-Reymond : ( J ) 



Place a cotton reel on a table and pull the thread in such a 

 direction that would tend to unroll it. If the reel is light it will 

 be drawn forward without rotating. If, on the other hand, it is 

 heavy, it will rotate without moving forward, and the thread 

 will be unwound. 



The couple of rotation vanishes when the hand is in supination. 

 In this position the muscular movement is exercised in the most 

 simple and efficient manner, especially if a great effort is called 

 for. 



The reader will remember that the finger joints are the only 

 articulations which can, with entire accuracy, be described as 

 having a single axis ( 75). The elbow joint, for example, is an 

 oval articulation and belongs rather to the second degree. 



Chauveau ( 2 ) has shown that the flexor muscles operate more 

 economically than the extensors, the consumption of oxygen 

 being less for the same work. He proved this for the muscles of 

 the forearm. Otto Fischer appears to have reached the same 

 conclusion as the result of certain experiments on the movements 

 of the legs. 



In the case of joints which are spherical or have three axes, 

 there can, of course, be several simultaneous movements, such as 

 flexion, or extension (in the sagittal plane), adduction (in the frontal 



(*) R. du Bois-Reymond, Spezielle Muskelphysiologie (loc. cit.) 

 (>) A. Chauveau (Comptes Rendus Sciences, 6th October, 1904). 



