278 VERTEBRATE SKELETON 



shown by the course of the nerve in figure 301) and another twist in the 

 antebrachium, radius and ulna crossing each other when the hand is 

 prone (fig. 315), so that, as may be seen in man or other plantigrade, 

 the digits point forwards. The condyles of the lower end of the 

 humerus are also involved in the twisting. 



The chief movement of the paired fins of fishes is a fanning of the 

 water, and largely in a single plane. In Tetrapoda other and 

 different kinds of motion are called for, resulting in a change in the 



articulation of appendage 

 and girdle. Instead of a 

 simple hinge, there is usually 

 a ball and socket joint, per- 

 mitting motion in almost 

 every plane. There has also 



Fig. 301. — Torsion in developing human arm , , , •. r 



(Braus), r, radius; m, ulna. Dotted line, course developed Capacity lor morC 



of radial nerve. ^j. jggg rotation of the distal 



parts, provided for by the two zeugopodial bones. The numerous 

 joints of the autopodium accommodate the foot to unevenness of 

 the ground and adapt it for numerous other purposes. 



The difference in the uses of fore and hind limbs accompany 

 changes in their relative size. In walking, the hind limb is flexed 

 and thrust forwards, the foot placed on the ground, after which 

 straightening of the hmb pushes the body forwards. On the other 

 hand the fore limb is straightened, extended forwards, and when 

 the foot is on the support, flexure of the limb pulls the body along. 

 Thus the hind leg is the more effective organ of terrestrial locomotion 

 and this has resulted in its greater size, except in those forms 

 (Ichthyosaurs, whales, etc.) which have reverted to the water 

 or which (Pterodactyls, birds, bats) have taken to flight. 



The feet are applied to the ground in different ways. The most 

 primitive is where the metacarpals or metatarsals and digits rest 

 on the surface (plantigrade). In digitigrades the mesopodial bones 

 are lifted, and in unguligrades the weight is supported on the tips 

 of the digits. Intermediate conditions between these three occur. 



The skeletal parts of the appendages may be reduced in other 

 ways than by fusion. In these reductions the more distal parts are 

 first to disappear, the process proceeding proximally, digits being 

 affected before mesopodial or basipodial parts. It is impossible to 

 say there is any universal law in order of reduction other than this, 



