THE STRUCTURE OF MAN 



lumbo-sacral boundary may be shifted back a vertebra, and in 

 the Chimpanzee even two vertebrae. 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 EIBS AND STERNUM 



Two types of variation of .the thorax are to be distinguished 

 in Mammals, a primary and a secondary type. The former is 



A B 



FIG. 22. A, TRANSVERSE SECTION OF THE THORAX OF A LOWER MAMMAL (OR OF 



THE HUMAN EMBYRO) ; 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 



