OSTEOLOGY AND ARTHROLOGY 73 



What makes the meanings of these terms a httle con- 

 fusmg is that, in human anatomy, some authors consider the 

 backward movement of the humerus as extension, and the 

 forward movement as flexion ; in order to be able to 

 compare these movements to those that the femur executes 

 in relation to the pelvis. 



Now, in our opinion, the indication of this correspondence 

 is not absolutely necessary ; since it ceases to be exact if 

 we wished, from the point of view of the direction given to 

 other segments of the skeleton, to establish the same relation 

 between the elbow and the articulation of the knee. 



It is therefore indispensable, when discussing quadrupeds, 

 to discontinue these terms, in order the more readily to 

 recognise that : in flexion the inferior extremity of the 

 humerus is directed backwards ; in extension, on the con- 

 trary, it is directed forwards. In the first case the humerus 

 approaches the scapula ; in the second, on the contrary, it 

 moves away from it. 



These movements, which take place during walking, are 

 executed in the following manner : When one of the anterior 

 limbs is at the end of that stage of progression which is called 

 support (see p. 289, Displacements of the Limbs) — that is to 

 say, during the time that the foot remains in contact with the 

 ground, whilst the trunk is moving forward — the direction of 

 this limb becomes more and more oblique downwards and 

 backwards. At a certain moment the limb is raised from 

 the ground, to be carried forwards, in order to be again 

 pressed on the ground, and recommence a new resting stage. 

 In these different phases the humerus is flexed. But at 

 the moment that the limb, when carried forwards, is about 

 to resume its contact with the ground it becomes directed 

 obliquely downwards and forwards ; then the humerus is in 

 the position of extension. 



During these movements of the humerus, there exists an 

 essential factor — that is, the scapular balance. (It is the 

 same as what occurs in man when he balances his arm in 

 the antero-posterior plane.) When the humerus is flexed, 

 the scapula moves in such a way that the superior portion 

 projects forwards ; when it is extended, the scapula, on the 



