742 ARTIODACTYLS xxxi. i 



The vertebral column shows the characteristics of other large 

 mammals in the development of high thoracic spines. Some of the 

 heavier types have a long rib series and graviportal 'single girder' 

 structure, but the tendency has been to retain and develop the break 

 in structure of the vertebral column behind the thoracic region, giving 

 a long lumbar region with forwardly directed transverse processes. 

 In rabbits and other mammals this division of the column is associated 

 with the jumping habit, and this is also found, though in a different 

 form, in ruminants (Young, 1955, p. 139)- Associated with this method 

 of progress is a fore-and-aft elongation of the pelvic girdle, the 

 ischium being well developed for the attachment of the retractor 

 muscles of the thigh. In making the jumping movements, which are 

 common in all artiodactyls and are especially used by the mountain- 

 loving types, the extensor muscles of the back (sacrospinalis and 

 multifidus) work with the retractors of the two hind limbs to give a 

 powerful thrust. 



The characteristic of the limbs is, of course, the equal development 

 of digits III and IV, with reduction of the rest. The gait was at first 

 plantigrade, then digitigrade; hoofs, differing from those of perisso- 

 dactyls, have developed on the toes. The elongation of the lower seg- 

 ments of the limbs and shortening of the upper has been similar to 

 that of perissodactyls, but the long metapodials have become united 

 in later forms to make the 'cannon bone'. The ulna and fibula become 

 reduced, as in horses. The presence of two digits has led to the reten- 

 tion of two bones in the distal row of carpals, the hamate and fused 

 magnum-trapezoid, and these articulate in interlocking fashion with 

 the three proximal carpals (Fig. 466). Similarly in the foot the two 

 lateral cuneiforms are fused to thrust upon the third digit, while the 

 fourth sends its thrusts to the cuboid and the latter is fused with the 

 navicular. Between this compound bone and the talus there is a very 

 characteristic joint, the under surface of the talus being grooved like 

 its upper surface. These joints of the carpus and tarsus are evidently 

 an important part of the apparatus of locomotion ; probably in both 

 limbs they serve to take strain when the animal is moving over un- 

 even ground, and in the leg they are also the seat of a considerable 

 propulsive thrust from the calf-muscles. In walking, the limb of artio- 

 dactyls is moved as a whole at the shoulder and hip, by action of the 

 upper muscles. The wrist and ankle joints bend just enough to raise 

 the feet off the ground, and the elbow and knee joints, lying so high 

 as to be hardly visible externally, also bend little. The essence of 

 artiodactyl locomotion is the use of the upper limb muscles; indeed 



