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, 7.e. between the twenty- 
fourth and twenty-fifth vertebre of the whole column. ‘This 
later assimilation anteriorly of sacral vertebree 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 vertebrz 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." This change is accompanied in Man by the 
depression of the promontory, which becomes duplicated (Fig. 21, 
C’ 0"). 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 (lv. 
of Figs.). 
In contrast to this reduction of the lumbar vertebrez 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 vertebre. This has 
become the normal condition in the Gibbon (Hylobates). 
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 (Lranchiosaurus) a distal shifting of the pelvic arch along six 
to seven vertebre 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 
(z.e. it does not become co-ossified with the other vertebre 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 vertebre 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. R. Dublin Soc., vol. v., Ser. 2, 
p. 123.] : 
