xi FORE-LIMB 501 



projects backwards. The collar-bone or clavicle is never 

 strongly developed in Mammals in which the fore-limb only 

 moves in one plane forwards and backwards : in the 

 rabbit it is a small, curved, rod-like bone, attached by 

 fibrous tissue at one end to the sternum and at the other 

 to the coracoid process of the scapula, there being small 

 cartilages at either end of it. 



The relative positions of the bones of the f OPG-limb are at first sight 

 somewhat difficult to understand owing to their having become altered 

 in the course of development. In your own fore-arm the bones can 

 be rotated on one another, so that the thumb can be made to point 

 outwards or inwards ; while in the rabbit the first digit has permanently 

 the same position, pointing inwards. To understand this, extend your 

 arm outwards with the thumb pointing away from the ground. The 

 back of the hand and arm, continuous with the dorsal surface of the 

 body, or back, is its dorsal surface ; the palm of the hand, and the sur- 

 face of the arm continuous with the chest, is its ventral surface : the 

 border of the arm and hand continuous with the thumb is the pre- 

 axial border ; and that continuous with the little finger the postaxial 

 border. This position is called the position of supination ; if 

 the fore-arm and hand be now rotated, so that the thumb points 

 inwards, the position is that of pronation. While in this position, 

 bend the elbow at right angles and bring it inwards close to the body ; 

 the preaxial border of the hand will now be on the inner side, and 

 an examination of the bones of the fore-arm shows that they cross 

 one another. It is in this position that the bones of the rabbit's fore- 

 limb are permanently fixed (Fig. 130, and compare Fig. 8). 



The proximal extremity of the humerus bears a rounded 

 head for articulation with the glenoid cavity, in front of 

 which is a groove for the tendon of the biceps muscle 

 (p, 61); certain tuberosities for the attachment of mus- 

 cles will also be observed. Its distal extremity presents a 

 large, pulley-like surface or trochlea for the articulation of 

 the bones of the fore-arm, and a deep depression or fossa, 

 perforated by a foramen, on its posterior side, for the recep- 

 tion of the end of the ulna. The radius is the shorter, 



