LIMB GIRDLES ^^o 



birds, where the whole thing is firmly bound to the thorax by 

 muscles. The scapula has a characteristic shape, and the prc- 

 coracoids are large and meet the sternum ; they are thus able to 

 resist the strong inward pull of the flight muscles (Fig. 315). Most 

 reptiles have a pelvic girdle with all three bones of unspecialised 

 type (Fig. 439). That of the birds (Fig. 315) is highly characteristic 

 and is strengthened for bipedal gait by a well-developed sacrum. 

 Thepubes are turned backwards, and there is no ventral symphysis. 

 In mammals the scapula is enlarged and the coracoid reduced 

 to a small bone which early in its development fuses with the 

 scapula to form a process which is said to resemble the head of a 

 crow, hence its name of coracoid. The clavicle is absent from or 

 much reduced in running forms, but well developed in types such 

 as man (Fig. 440) which can move their fore-limbs from side to 

 side. The interclavicle is probably the bone usually called omo- 

 sternum. All three bones are present in the pelvic region, but they 

 fuse to form a single os innominatum. 



POSITIONS OF LIMBS 



In accordance with the fin-fold theory the primitive position 

 of the paired limb was that of a flat plate projecting from the 

 body ; it would then have a dorsal and a ventral surface, and a 

 preaxial and a postaxial border, the axis here referred to being 

 that of the limb itself. A human being lying prone (i.e. with face 

 to the ground) will have his fore-limbs arranged in this position 

 if he stretches his arms out at right angles to the body with palms 

 to the ground. The palms are ventral, the radius and thumbs 

 preaxial. In most fish the fins have rotated so that the ventral 

 surface is posterior ; this can be demonstrated by turning the 

 hands so that the thumbs are below. This is also the position 

 in the primitive tetrapods. The later land animals, including 

 many living reptiles, have raised their body by a double right- 

 angled flexure of the limbs. Sta^jng from the primitive position 

 the upper arm remains in the same relation to the body, the 

 forearm bends down until it becomes vertical, and the hand 

 bends in the opposite direction at the wrist joint until its palm 

 is again horizontal and in contact with the ground. In most 

 mammals the humerus has rotated backwards and inwards so 

 that the elbow is a backwardly directed joint, and the forearm 

 and hand have rotated forwards and inwards to bring the radius 



