1 6 THE VERTEBRAL COLUMN. [chap. 



forwards as well as outwards, is usually thick and rounded, 

 and is nearly always in relation with the anterior zygapo- 

 physis, is termed vietapophysis ;^ the one placed rather lower 

 (Fig. 4, a), and which projects more or less backwards, and 

 is generally rather slender or styllform, is called anapophysis. 

 These, with the zygapophyses before mentioned, sometimes 

 called oblique processes, but which are rather articular surfaces 

 than true processes, are all the processes commonly met with 

 on any Mammalian vertebra. 



Dei'elopment of the Vei'tcbrce. — The first indication of the 

 formation of a vertebral column in the embryo is the 

 appearance of a longitudinal primitive dorsal groove in tlie 

 germinal membrane, the edges of which {lainince dorsales) 

 rise up and meet above, so as to convert the groove into a 

 canal. From the tissue lining this canal (uppermost layer 

 of the germinal membrane) the brain and spinal cord are 

 developed, and in its walls are formed anteriorly the cranium, 

 and posteriorly the vertebral column ; the canal itself be- 

 coming the cerebral cavity and the neural canal of the spine. 



In the floor of this canal, formed by a horizontal lamina 

 which separates it from another and larger, ventral or haemal 

 canal (formed by the approximation in the middle Hne below 

 of the lamincB ventj-ales), a slender rod of peculiar structure 

 is developed. This is the notochord or chorda dorsalis, 

 around which the bodies of the future vertebrae are deve- 

 loped. In the MammaHa it almost completely disappears at 

 a very early period, traces only remaining in the axis of the 

 intervertebral substance, though in many of the inferior 

 Vertebrata it is persistent as a continuous rod for a longer 

 period, and sometimes permanently. 



^ It is also called " mammillary process" in some works on Human 

 Anatomy. 



