ATAVISM OR REVERSION. 135 



Examples of each form will now be considered. The 

 attainment of a functional condition by parts normally 

 suppressed is well illustrated in the case of man by 

 supernumerary ribs. Normally the number of ribs on 

 each side is twelve : not infrequently the number is in- 

 creased by an additional rib at the cervical or lumbar end 

 of the series. In many birds and reptiles all the cervical 

 vertebrae bear ribs ; in mammals the cervical ribs are 

 represented in a vestigial form, and in man such vestiges 

 are confined to the fifth, sixth, and seventh vertebrae. 

 Occasionally the rib attached to the seventh cervical ver- 

 tebra acquires a length of five to seven centimetres, becom- 

 ing perceptible, to practised eyes, even in the living body. 

 Supernumerary ribs attached to the lumbar vertebrae are 

 more instructive than those in the neck. Each normal 

 rib has inserted into its angle a fan-shaped muscle which 

 raises it during inspiration and is known as the levator 

 costce. In the lumbar region these muscles are repre- 

 sented by fan-shaped collections of tendons connected 

 with the transverse processes of the vertebrae. In many 

 instances, when a lumbar rib is present, the correspond- 

 ing tendinous levator is replaced by a functional muscle. 



This is instructive, as it illustrates a fact which must be 

 constantly borne in mind when investigating suspected 

 cases of reversion, viz., atavistic parts do not belong- to 

 forms pal&ontologically remote or systematically far dis- 

 tant. This is particularly insisted upon by Gegenbaur. 



In the present case the lumbar ribs are represented by 

 extra centres of ossification at the extremities of the 

 transverse processes ; normally they fuse with these 

 processes, but occasionally attain a functional size 



