60 ANATOMY OF THE RABBIT. 
or in related primates, they tend to form an obtuse angle which 
is progressively reduced from lower to higher types. 
It will be evident from a study of the limb skeleton of the 
rabbit that there is a general correspondence in structure as between 
its anterior and posterior divisions. This applies 
not only to the distinction 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 as between 
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 LIMB 
SKELETON. 
The pelvic girdle is more uni- 
formly 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 strong attachment to =the 
\ sacrum. On the other hand, the 
pectoral girdle is notable chiefly for 
the strong development of the 


Fic. 32. Plan af the anterior limb skele- 
ton in walking vertebrates, the equivalent 
elements of the posterior limb indicated in 
brackets: si, scapula (ilium); pp, procora- 
coid (pubis); ci, coracoid (ischium); hf, 
humerus (femur); rt’, radius (tibia); 
uf’, ulna (fibula); rt, radial carpal (tibial 
tarsal); i, intermedium; uf, ulnar carpal 
(fibular tarsal); c,c, centrals; I-5, distal 
carpals (tarsals); m. metacarpals (meta- 
scapula as opposed to the weakness 
of the ventral, pectoral portion. 
In the rabbit the clavicle is at best 
vestigial, and in many mammals, 
such as the ungulates, it is entirely 
absent. In man, however, as well 
tarsals); ph, phalanges of the digits. 6 
as in other mammals, many of 
them primitive, in which the pectoral muscles of adduction are well 
developed, together with the corresponding movement of shoulder 
and arm, the clavicle is both well developed and articulated at the 
two ends with the scapula and sternum. 
Both pectoral and pelvic girdles in terrestrial vertebrates 
conform more or less closely to a tri-radiate shape if the two halves 
