THE SPINAL SKELETON. 



45 



class. It is possible, however, for the neural spine to send 



back a pair of processes (hyperapophyses '), 



as in Galago, &c., embracing the neural 



spine next below, or, as in Dolphins, that 



a pair of metapophyses may project pre- 



axically from one spine and embrace that 



of the next vertebra. These, however, do 



not support articular processes, and are 



rather checks than joints. 



But a much more complex mode of arti- 

 culation is possible. Thus, in Serpents and 

 Iguanas we may have a median prominence 

 with two articular surfaces, developed from 

 the pre-axial surface of the neural arch, and 

 fitting into a corresponding concavity on the 

 post-axial surface of the vertebra in front. 



This pre-axial wedge-shaped process is 

 called the zygosphene? and the "correspond- 

 ing post-axial excavation is termed the 

 zygantrum. 3 



The maximum of dorsal joint complica- 

 tion, however, is found in the last dorsal 

 vertebra of certain Edentates, e.g. the Great 

 Ant-eater. Here each postzygapophysis 

 develops two additional articular surfaces, 

 one on each side of a notch, which receives 

 a process from the pre-axial side of the 

 neural arch, which process is furnished with 

 two corresponding surfaces there thus 

 being three articular surfaces on each side 

 of such vertebra, fitting into corresponding 

 surfaces of the vertebra adjacent in the 

 mode known in carpentry as " tenon and 

 mortice." 



In so far as man's dorsal vertebrae are all 

 free (i.e. true vertebrae), he agrees with other 

 Vertebrates, except such abnormal forms as 

 Chelonians and the Glyptodon, and except 

 also Birds, in which anchylosis unites a 

 greater or less number of the vertebrae next 

 the lumbar region. 



C 



rl 



1 From vvfp, beyond, over, and apophysis. 

 - From ^vfitv, a yoke, and <r<pi]t , a wedge. 

 3 From vi6v, and avrpov, a cavity. 



FIG. 58. SPINAL 

 COLUMN OF 

 GALAGO. 



2, postzygapophyses ; 

 /;, hyperapophyses. 



