chap, in.] VERTEBRA AND RIBS: MAN. 107 



gion, though differing widely in different classificatory divisions, 

 are as a rule maintained with some constancy within the limits of 

 those divisions, which may be species, genera or larger groups, so 

 that vertebral formulae are often of diagnostic importance. Changes 

 in the numbers of vertebrae composing the several regions must 

 therefore have been an important factor in the evolution of the dif- 

 ferent forms. 



Homceotic Variation in the spinal column consists in the as- 

 sumption by one or more vertebrae of a structure which in the 

 type is proper to vertebrae in a different ordinal position in the 

 series. Examples of this are seen in the case of the development 

 of ribs on a vertebra which by its ordinal position should be lum- 

 bar ; or in the occurrence of a vertebra, normally lumbar, in the 

 likeness of a sacral vertebra, having its transverse processes modi- 

 fied to support the pelvic girdle, &c. Variations of this kind have 

 one character in common, which though at first sight obvious, will 

 help us in interpreting certain other cases of Homoeosis. In all 

 cases of development of a vertebra normally belonging to one region, 

 in the likeness of a vertebra of another region, this change always 

 takes place in vertebrae adjacent to the region whose form is as- 

 sumed. For example, if one vertebra, normally cervical, bears ribs, 

 it is always the last cervical ; if two cervicals bear ribs, they are the 

 last two, and so on. No gaps are left. 



Homceotic Variation in the spinal column may occur by the 

 assumption of 



(1) dorsal characters by a vertebra in the ordinal position of a 

 cervical, 



(2) lumbar characters by a vertebra in the ordinal position of a 

 dorsal, 



(3) sacral characters by a vertebra in the ordinal position of 

 a lumbar, 



(4) coccygeal characters by a vertebra in the ordinal position of 

 a sacral, 



or by the reverse of any of these. Since almost any of these 

 changes may occur either alone or in conjunction with any of the 

 others, it is not possible to group cases of such Homoeosis under 

 these heads, but the consideration of the more complex cases will 

 be made easier if simple examples of each class are first described 

 as seen in Man. 



I. Simple cases. — Man. 



(1) Homoeosis between cervical and dorsal vertebrce. 



(a) From cervical towards dorsal type. 



The chief character distinguishing dorsal vertebrae is the pos- 

 session of moveable ribs. This character may to a greater or less 

 extent be assumed by cervicals. 



