236 THE UPPER LIMB IN MAN. 



the animal ; and the anteroposterior in man not only becomes 

 an inward rotation, but is much diminished in extent in the 

 apes, and is all but eliminated in the typical quadruped. 



6. — The Arm. 



a. The shaft of the upper limb in man, like that of the 

 lower limb, is much more harmoniously developed as regards 

 their lengths respectively to one another, to the length of the 

 trunk, and to the lengths of thigh to leg, and of arm to fore- 

 arm, than in the animal. The shafts of the human limbs are 

 longer in relation to the trunk than in the animal. 



b. In the pendulous position of the human arm, its axis 

 is at right angles to the axis of the shoulder. This may be 

 termed the vertical position, or position of passive flexion. 

 By muscular action it may be carried before or behind the 

 trunk to about 30° beyond its vertical position; in such posi- 

 tions it is in full flexion. It cannot be extended appreciably 

 beyond the level of the axis of the shoulder without moving 

 the scapula. From its fundamental extended position, in 

 which it is in a line with the axis of the shoulder, it can be 

 swept in rotation horizontally forward to about 30° nearer 

 the mesial plane than its vertical position, and to the same 

 extent in successive depressions or degrees of flexion, so far as 

 the interposition of the trunk will permit, on to extreme 

 flexion. 



c. No animal can fully extend the shoulder-joint — that is, 

 bring it in a line with the axis of the shoulder. The amount 

 of deficiency in flexion is equal to the angle of antero-posterior 

 obliquity of the shoulder. And again, in proportion to the 

 increase in transverse obliquity, is the limitation of the move- 

 ments of the shoulder at right angles to the plane of flexion 

 and extension. 



d. The distinct specialisation, and the great development 

 of the deltoid in man, is in direct relation to the perfection 



