34 THE STRUCTURE OF MAN 



These changes come to an end when the twenty -fifth 

 vertebra, by virtue of its apposition with the hip-girdle, becomes 

 the first sacral, and the promontory attains its full differentiation 

 between it and the last lumbar vertebra, i.e. between the twenty- 

 fourth and twenty-fifth vertebrae of the whole column. This 

 later assimilation anteriorly of sacral vertebrae is further evident 

 in the fact that synostosis between the separate parts of the 

 sacrum always takes place from behind forwards. 



The tendency of the human pelvic girdle to extend even 

 farther forwards is revealed, in cases in which the last or fifth 

 lumbar vertebra enters into the constitution of the sacrum. The 

 number of pre-sacral vertebrae is in such a backbone reduced to 

 twenty-three, and this is the normal condition in the Orang and 

 Chimpanzee, and the general, though not the invariable, condition 

 in the Gorilla. 1 This change is accompanied in Man by the 

 depression of the promontory, which becomes duplicated (Fig. 21, 

 C' C"). The sacrum appears deeply sunk into the pelvis ; 

 although such sinking may also occur, as is shown in Fig. 21, 

 A! A" without any incorporation of the fifth lumbar vertebra in 

 the sacrum. In both cases the iliac crests rise almost to a level 

 with the upper edge of the penultimate lumbar vertebra (l.iv. 

 of Figs.). 



In contrast to this reduction of the lumbar vertebrae to four, 

 the shifting of the pelvic girdle during development may be 

 arrested one vertebra behind the normal ; in such cases, which 

 are rare, we have twenty-five pre-sacral vertebrae. This has 

 become the normal condition in the Gibbon (Hylolates}. 



Similar variations are found in individual Orangs, Gorillas, 

 and Chimpanzees. In the Orang and Gorilla, for instance, the 



direction. Credner, by comparing young with old specimens, has proved that 

 in a fossil Amphibian (Branchiosaurus) a distal shifting of the pelvic arch along six 

 to seven vertebrae took place ontogenetically. 



1 [In this animal, the last lumbar vertebra, although it may take on the relation- 

 ships and detailed structure of a sacral vertebra, always retains its independence 

 (i.e. it does not become co-ossified with the other vertebrae of the sacral series as in 

 the Orang and Chimpanzee). The presence of a highly differentiated articulation 

 between the last lumbar vertebra and the anterior border of the ilium is an invariable 

 characteristic of certain Armadillos. The joint thus formed is a transverse one, which 

 comes into especial use when the animal rolls itself up, and is therefore of a purely 

 adaptive nature. It is well to guard against confusion between this condition and 

 that of incorporation of lumbar with sacral vertebrae under extension or forward 

 translocation of the hip-girdle, in which the extra articulation is a longitudinal 

 one lying on the inner border of the iliac head. (Cf. Symington, Jour. Anat. 

 and Phys. vol. xxiv. p. 42. ; and Paterson. Trans. It. Dublin Soc., vol. v., Ser. 2. 

 p. 123.] 



