150 HUMAN EMBRYOLOGY AND MOEPHOLOGY. 



vertebra fuse and give rise to the condition shown in Fig. 119 C. 

 In the 7th week two centres appear in the body, but quickly fuse ; 

 one appears in each neural arch (8th week) ; at birth the ossific 

 centres of the body and neural arch have met. The central and 

 neural ossifications meet at the neuro-central suture and unite at 

 the 4th or 5th year. The neural ossifications fuse behind (where 

 the spinous process is produced) in the 1st year. The ribs are 

 entirely supported from the neural ossifications (Fig. 119 B). The 

 spinous and transverse processes are formed by outgrowths of 

 cartilage into the septa between the proto- vertebrae or primitive 

 segments. The ribs are also formed in the septa, each at first 

 articulating with a vertebra, which is also inter-segmental in 

 origin. In all the ribs, except the 1st, 11th and 12 th the head 

 is shifted to the inter-vertebral disc in front of its vertebra. 

 Epiphyseal centres for the ossification of the transverse and spinous 

 processes appear about puberty. 



The Bodies of Mammalian Vertebrae are peculiar (1) in the 

 development of an upper and lower epiphyseal plate ; (2) in that 

 no trace of the notochord remains within them. In Fishes, as in 

 the early human or mammalian foetus, the bodies are hour-glass 

 shaped (amphicoelous, Fig. 119 F) ; in Amphibians they may 

 retain a concavity in front (procoelous) or behind (opisthocoelous), 

 but in mammals both ends are filled up. 



, 1st Cent infith D. Vert 



M:|:l:|.M.h».'lllirifli.|.|.l.|«|.i-|.|:|:|:|-l I 



Axis in 4th month < — 7th week > 5th Sac. in 5th month 



1st cent in Atlas. 



r;l:l:l:l:l 



7th week > 3rd Sac. in 7th month 



Fia. 120.— The Order in which the Centres of Ossification appear in the Bodies (A) 

 and in the Neural Arches (B) of the Spinal Column. 



It will be observed (Fig. 120 B) that the centres of ossification 

 for the neural arches appear first in the anterior end of the spine 

 (1st cervical), the date becoming later the more posterior the 

 vertebra. In the 1st sacral they appear about the 4th month;. 



