84 Wdlter Staj)ley : 



de;jreneration " throws no light on the cause of cervical ribs, but as 

 the development of cervical ribs in uianinials is associated with 

 degenerative changes in the neck, the term is useful. 



Considerable confusion seems to exist about vestiges and cervical 

 ribs. It is as well to preface the remarks on this subject by quota- 

 tions from Morris's Human Anatomy and Arthur Keith's Huuum 

 Embryology and Mor])holog}'. 



Morris says: — "The costo-transverse foramen is very characteristic 

 of a cervical vertebra. It is bounded internally by the pedicle, pos- 

 teriorly by the transverse process?, which corresponds to the trans- 

 verse process of a thoracic vertebra, anteriorly by the costal process, 

 which corresponds to the rib in the thoracic vertebra, and externally 

 by the costo-transverse lamella. 



'■ Tlie transverse process (of the seventh cervical vertebra) is mas- 

 sive ; the costal element of the process is very small, but, on the 

 other hand, the posterior or vertebral part of the process is large, and 

 becoming nioie like the transverse process of a dorsal vertebra. The 

 costo-transverse foramen is the smallest of the series and may be 

 absent. Occasionally the costal process is segmented off and con- 

 stitutes a cervical rib.'' 



Keith says: — "Vestigial Ribs: Although the ribs are only fully 

 developed in the dorsal region, yet a representative, a costal element, 

 is present in every vertebra." 



" The costal process of the seventh cervical, usually represented by 

 a mere vestige, may develop into a rudimentary or even a fully 

 formed rib." 



It is difficult to accept this teaching of Keith, because it contains 

 an error somewhat akin to the exploded idea that cranial bones are 

 modified vertebrae, for he confuses the costal process, which is a 

 reduced rib, with the costal rudiment, from which all ribs must 

 develop. 



The formation of costal processes is shown in the monotremes : 

 there, in the PlatypuK, the rib-stumps are separated from the verte- 

 brae by joints, but in the Echidna these joints are more or less 

 indistinct and the ril) tissue is l)econiing confluent with the neck 

 bones. In the higher mammals these joints have entirely disappeared 

 and the suppressed ribs remain as costal processes or rib vestiges. 

 Costal processes are ve.stiges of ribs because they mark the remains 

 of pre-existing cervical ribs. Mannnalian cervical ribs are develop- 

 ments because they appear in positions which have been occupied 

 cither by ribs that have become extinct, or by costal processes, ribs 

 that have become vestigial. Cervical ribs are developed ribs, and 

 the development varies from slight to full. A developing, or a 

 developed, structure is not a vestige, nor is a vestige a rudiment, nor 



