THE SPINAL COLUMN AND BACK. 



147 



chord is enclosed in the suspensory ligament. The body of each 

 vertebra is formed round the notochord and at first each has 

 an hour-glass canal surrounding the notochord. Within each 



pituitary 



~ — basi-sphen. 



basi-occip. 



suspensory lig. 

 axis. 



interuert. disc, 

 centre of disc 



fr- C 



-coccyx. 



Fig. 117. — Where Remnants of the Notochord may occur in the Adult. 



vi 



body the notochord ultimately disappears but in the inter- 

 vertebral discs it swells out and forms much of the central 

 mucoid core which each disc contains. The notochord with its 

 membranous sheath is the earliest form of spinal column known. 

 The real vertebral column, formed out of its sheath, begins to 

 supplant it even in low vertebrates and in the human foetus of 

 the second month this change is also seen to take place. 



Proto-vertebrae or Primitive Segments. — Proto-vertebrae 

 are not the forerunners of the vertebrae ; they are the primitive 

 segments into which the mass of mesoblast at each side of the 

 neural canal and notochord divides (Figs. 233, p. 289 and 116). 

 The process of division or segmentation begins at the occipital 

 region towards the end of the second week and spreads backwards 

 until 3 5 or more body segments or somites are cut off. Each 

 segment thus separated forms its own muscles (from its muscle 



