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 
vertebre 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 vertebre. 
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 vertebre 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 /evator 
coste. In the lumbar region these muscles are repre- 
sented by fan-shaped collections of tendons connected 
with the transverse processes of the vertebra. 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 paleontologically 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 
