60 ANATOMY OF THE RABBIT 



Since the chewing motion of the mandible is almost entirely 

 antero-posterior, these ridges are at right angles to the direction 

 of movement in this activity. 



The Humaft Skull 

 If the human skull be compared with that of the rabbit or other 

 mammal, it is seen to differ most markedly in the enormous de- 

 velopment of the cranial region, and in the anteroposterior com- 

 pression of the face, with which is associated shortening of the 

 jaw region, reduction of the nasal cavities, and rotation of the 

 orbits to a forward position. A most instructive feature is the re- 

 adjustment of the axes of the skull, coincident with the assumption 

 of the erect position. In both quadrupedal and bipedal positions 

 the face naturally retains its forward direction. In most mammals, 

 as in vertebrates generally, the axial line of the cranium, known as 

 the basicranial axis, and that of the face, the basifacial axis, tend 

 to be nearly in a straight line or at least parallel ; while in primates 

 they tend to form an obtuse angle which is progressively reduced 

 from lower to higher types, being smallest in man, where it 

 approximates a right angle. 



The Appendicular Skeleton 



It will be evident from a study of the limb-skeleton of the rabbit 

 that there is a general correspondence in structure between its 

 anterior and posterior divisions. This not only applies to the dis- 

 tinction of girdle portions and the divisibility of the free extremity 

 into proximal, middle, and distal portions, but extends to very 

 many smaller details of composition. The relation in structure be- 

 tween anterior and posterior limbs is described as serial homology, 

 since two structures in the same animal cannot be homologous 

 in the usual meaning of the term. 



The pelvic girdle is more uniformly and solidly developed than 

 the pectoral girdle, as shown by its strength in the three principal 

 directions about the point of attachment of the limb, the great 

 development of the ventral union, and the strong attachment to the 

 sacrum. These features correspond with the usually proportionally 

 greater thrust of the hind limbs in support of the body-weight and 

 in locomotion. On the other hand, the pectoral girdle is notable 



