3 i8 COMPARATIVE ZOOLOGY OF CHORD ATES 



in the dog. In the more primitive forms, and those which are 

 specialised for tree-climbing and digging, the clavicle is usually 

 present. 



The limbs themselves show interesting modifications. In 

 the earliest Tetrapods, the limbs stuck straight out at right 

 angles to the side of the body. When the ventral surface of 

 the body became lifted off the ground, the upper arm and thigh 

 stuck straight out laterally and horizontally ; at the elbow and 

 knee there was a right-angle bend, so that the forearm and 

 shank descended vertically to the ground. At the wrist and 

 ankle, there was another right-angle bend, so that the hand and 

 foot extended horizontally away from the body. 



In the mammals, starting from the condition just described, 

 the limbs have undergone a rotation. The hind limbs have 

 been rotated forwards, so that the thigh runs forwards from the 

 hip-girdle, and parallel with the side of the body, the shank 

 runs downwards, and the foot points forwards again. In 

 the fore limb, however, the upper arm has been rotated back- 

 wards parallel with the side of the body, and the forearm runs 

 downwards. But the hand would point backwards if the fore 

 limb had undergone a simple rotation similar to that of the 

 hind limb (though in the opposite direction). As a matter of 

 fact, the hand points forwards, and this is brought about by a 

 rotation of the wrist through 180 about a vertical axis which 

 coincides with the forearm. So it happens that the forearm 

 is twisted, and the radius runs from the outer side of the elbow 

 to the inner side of the wrist, passing in front of the ulna, which 

 runs from the inner side of the elbow to the outer side of the 

 wrist. This is the typical position (of pronation) in mammals ; 

 most Primates, including man, however, are able to uncross the 

 radius and ulna and so turn the palm of the hand upwards 

 (supination). 



It is impossible to go into all the types of limb-structure, 

 but it is interesting to consider the adaptations of limbs to the 

 three great media, viz. to locomotion on land, in the air, and 

 in water. 



The fingers and toes of land-living vertebrates above the 

 amphibia end in horny claws which may be modified into 



