xi FORE-LIMB 501 



clavicle is never strongly developed in those 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 fore-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 surface of the arm continuous 

 with the chest, is its ventral surface ; the border of the arm 

 and hand continuous with the thumb is the preaxial 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 examina- 

 tion 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 hnmerns bears a 

 rounded Jiead for articulation with the glenoid cavity, in 

 front of which is a groove for the tendon of the biceps 

 muscle (p. 61) ; certain iiiberosities for the attachment of 

 muscles 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 reception of the end of the ulna. 

 The radius is the shorter, inner (preaxial) bone of the 



