602 EMBRYOLOGY. 



The rudimentary fundaments of the cervical ribs at their first 

 appearance fuse with the cervical vertebrce, at one end with the body of 

 the vertebra, at the other with an outgrowth of the neural arch, 

 and with the latter enclose an opening through which the vertebral 

 artery runs the foramen transversarium. The so-called transverse- 

 process of the cervical vertebra is therefore a compound structure, 

 and were better designated lateral process, for the bony rod that lies 

 dorsad of the foramen transversum is formed by an outgrowth from 

 the vertebra and alone corresponds to the transverse process of a 

 dorsal vertebra ; the ventral rod, on the contrary, is a rudimentary 

 rib, which possesses in fact a separate centre of ossification. 



The fundament of the rib of the seventh cervical vertebra occa- 

 sionally attains greater size, does not fuse with the vertebra which, 

 consequently does not possess any foramen transversarium and is 

 described under the abnormalities of the skeleton as free cervical rib. 

 Its presence is explained therefore as being the result of a more volu- 

 minous development of a part which in all cases exists as a fundament. 



The transverse process of the lumbar vertebrce is also better designated 

 as lateral process, because it encloses the rudiment of a rib. This ex- 

 plains the phenomenon of a thirteenth or small lumbar rib occasion- 

 ally observed in Man. 



The sacral region is the one that is most modified. A large number 

 of vertebrae in this region by becoming firmly united with the pelvic 

 girdle have lost the power of moving on one another, and are fused 

 together into a large bone : the sacrum. This consists in human 

 embryos of five separate cartilaginous vertebrae, the first three of 

 which especially are characterised by very broad, well-developed 

 lateral processes. 



I say lateral processes because comparative-anatomical grounds 

 and embryological evidence both indicate that there are included in 

 them rudimentary sacral ribs, such as in lower Vertebrates make their 

 appearance as independent structures. On the embryological side, 

 the method of their ossification favors this view, for each sacral 

 vertebra undergoes ossification from five centres. To the three 

 typical centres, those of the body and the neural arches, are added 

 in the lateral processes large bone-nuclei (centres), which are com- 

 parable with the centres of ossification of a rib. They produce th& 

 well-known lateral masses of the sacrum (niassse laterales), which, 

 bear the articular surfaces for union with the ilium. 



The fusion of the five bony pieces of a sacral vertebra, at first 

 separated by strips of cartilage, takes place later than in other parts- 



