SKELETON OF LOWER LIMB. 



195 



in man is the os cloaca:. It is an azygos median structure 

 which extends backwards behind the ischio-pubic symphysis. 

 as e.g. in the Lizard Psauimosaurns. 



Similarly a median azygos structure (pointed or forked) 

 may be developed from the front of the ischio-pubic sym- 

 physis, as in Menobranchus and Salamandra. 



The acetabuhun of man is very well developed as compared 

 with that of other animals. In its formation by the three 

 pelvic bones it follows the universal mammalian rule. In that 

 its cup is completely ossified it also 

 resembles all Mammals except the 

 lowest (i.e. the Monotremes), which 

 agree with Birds in having the ace- 

 tabula perforated. 



In Reptiles generally three separate 

 ossifications also concur to form the 

 acetabulum. In Crocodiles, however, 

 the so-called pubis does not concur 

 in its formation. 



12. The supremacy in size of man's 

 FEMUR over the other parts of his 

 skeleton is not shared by the femur of 

 most other Vertebrates, and this im- 

 portant bone seems rarely, if ever, to 

 have any representative whatever in 

 the skeleton of Fishes. Nevertheless 

 a single cartilage (as in Ceratodus] 

 which articulates with the limb root 

 may be the representative either of 

 the thigh or leg bones. 



Confining ourselves therefore, for 

 purposes of comparison, to Mammals, 

 Sauropsida, and Batrachians, we find 

 the femur under a certain aspect more 

 constantly present than the humerus. 

 For although it is often absent when 

 the humerus is present (as in forms like Siren, which have 

 pectoral limbs but no pelvic ones), yet it is sometimes present 

 in a more or less rudimentary condition when no representa- 

 tive of the foot coexists with it. 



Such is the case, e.g., in some Whales (as the Greenland 



Whale) amongst Mammals, and certain Snakes, e.g. Boa, and 



certain Lizards, e.g. Lialis, amongst the Reptiles. In absolute 



length the femur of man exceeds that of almost all other 



O 2 



FIG. 167. CARTILAGINOUS 

 SKELETON OF A LIMB OF 

 Ceratodus. (After Giin- 

 ther.) 



The large upper piece articu- 

 lates with the limb root. 



