7LOP 
36 THE STRUCTURE OF MAN 
lumbo-sacral boundary may be shifted back a vertebra, and in 
the Chimpanzee even two vertebre. In the former case the 
position normal to Man is attained. 
It is evident that shifting of the pelvic girdle (and, as will 
be seen later, of the pectoral girdle also) cannot take place 
without concomitant variations in other organs. To this question 
we shall return. 
THE RIBS AND STERNUM 
Two types of variation of the thorax are to be distinguished 
in Mammals, a primary and a secondary type. The former is 
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Seidl Z 
2s 2 
IU: se 
A B 
Fic, 22.—A, TRANSVERSE SECTION OF THE THORAX OF A LOWER MAMMAL (OR OF 
THE HUMAN EMmByRO); B, THE SAME OF A MAN, 
In the former it is the vertical diameter which is the greater, in the latter 
it is the transverse, as indicated by arrows. 
far more common than the latter, and is found in most Mammals, 
including the lower Apes. The thorax of this primary type (Fig. 
22, A) is elongated, its dorso-ventral greatly exceeding its trans- 
verse diameter (carinate or keeled type). 
The secondary type (Fig. 22, B) is found in Anthropoid 
Apes and in Man. The dorso-ventral diameter is here greatly 
diminished and the transverse is increased in proportion; the 
broad thorax is somewhat barrel-shaped, and often compressed 
antero-posteriorly. This secondary type is preceded, both onto- 
genetically and phylogenetically, by the primary. 
It is evident that the associated modifications, viz. the 
