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, 

 distal portions, but extends to very many smaller 



THE LIMB 

 SKELETON. 



middle 

 details 



and 



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 pelvic girdle is more unir 

 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 

 scapula as opposed to tl^e 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 

 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 



Fig. 32. Plan of 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) ; 

 u{', ulna (fibula) ; rt, radial carpal (tibial 

 tarsal); i, intermedium; uf, ulnar carpal 

 (fibular tarsal); c.c, centrals; i-s, distal 

 carpals (tarsals) ; m. metacarpals (meta- 

 tarsals) ; ph, phalanges of the digits. 



