20 



VERTEBRAE 



are separated by a certain amount of fibrous tissue forming 

 the intervertebral disc, and the apposed surfaces of the centra 

 are as a rule nearly flat. In this last feature, and in the 

 important fact that the centra are ossified from three distinct 

 centres, the anterior and posterior pieces (" epiphyses ") remaining 

 distinct for a time, even for a long time (as in the Whales), 

 the centra in the mammals differ from those of reptiles and 

 birds. The epiphyses are not found throughout the vertebral 

 column of the lowly-organised Monotremata, and they do not 

 appear to exist in the Sirenia. 



Fio. 5. Anterior surface of 

 Human thoracic vertebra 

 (fourth), x . az, Anterior 

 zygapophysis ; c, body or 

 centrum ; I, lamina, and 

 p, pedicle, of the neural 

 arch ; nc, neural canal ; t, 

 transverse process. (From 

 Flower's Osteology of the 

 Mammalia.) 



FIG. 6. Side view of first 

 lumbar vertebra of Dog 

 (Canis famttiaris). x $. 

 a, Anapophysis ; az, an- 

 terior zygapophysis ; w, 

 metapophysis ; pz, pos- 

 terior zygapophysis ; *, 

 spinous process ; t, trans- 

 verse process. (From 

 Flower's Osteology.) 



From each side of the centrum on the dorsal side arises a 

 process of bone which meets its fellow in the middle line above, 

 and is from there often prolonged into a spine. A canal is thus 

 formed which lodges the spinal cord. This arch of bone is 

 known as the neural arch, and the dorsal process of the same as 

 the spinous process. The sides of the neural arch bear oval 

 facets, by which successive vertebrae articulate with one another : 

 those situated anteriorly are the anterior zygapophyses, while 

 those on the posterior aspect of the arch are the posterior 

 zygapophyses ; these articular facets do not exist in the tail- 

 region of many mammals, e.g. Whales. 



In addition to the dorsal median spinous process of the 



