20 VERTEBRAE CHAP. 
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. 
Fia. 5.—Anterior surface of Fic. 6.—Side view of first 
Human thoracic vertebra 
(fourth). x2. az, Anterior 
zygapophysis ; ¢, body or 
centrum; J, lamina, and 
p, pedicle, of the neural 
arch ; 7c, neural canal ; ¢, 
transverse process. (From 
Flower’s Osteology of the 
Mammalia. ) 
lumbar vertebra of Dog 
(Canis familiaris). x 2. 
a, Anapophysis ; az, an- 
terior zygapophysis ; 1, 
metapophysis ; pz, pos- 
terior zygapophysis ;_ s, 
spinous process ; ¢, 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, eg. Whales. c 
In addition to the dorsal median spinous process of the 
