170 ELEMENTS OF ORNITHOLOGY. 
Various bony prominences projecting from the vertebra are 
termed processes. From the middle of the dorsal side of the 
neural arch there generally arises a prominence called the neural 
spine or neurapophysis, though it is sometimes simply called the 
spinous process. Prominences which jut out laterally from the 
side of the neural arch or centrum are called transverse processes. 
There may be two of them on either side. In such case the 
superior one is called the diapophysis or tubercular transverse 
process, and the inferior one the parapophysis or capitular 
transverse process. The distal ends of these two processes may 
be connected on either side by an osseous bridge, which may 
be prolonged into a process—the pleurapophysis. Two pro- 
cesses may arise, one from either side of the ventral surface of 
the centrum, and meeting together below form an arch beneath 
it comparable with the * neural arch” above. Such an inferior 
arch is termed a hemal arch, and serves to protect and transmit 
blood-vessels. 
A median process may depend from the middle surface of 
the centrum or the ventral end of the hemal arch. Such a part 
is named a hypapophysis, and it may bifurcate laterally, as in 
the Penguins. 
Two other processes generally project forwards and two 
backwards from the neural arch of each vertebra. These four 
are termed zygapophyses or articular processes. They are so 
termed because they serve to articulate adjacent vertebra toge- 
ther. Each articular surface of the anterior pair of zygapophyses, 
or prezygapophyses, looks more or less dorsally, or dorsad, and 
receives upon it the articular surface of one of the pair of 
posterior zygapophyses, or postzyyapophyses, the articular sur- 
faces of which Jook more or less ventrally, or ventrad. 
_ Adjacent vertebre are also articulated by the adjacent sur- 
faces of their centra. Parts which thus articulate have inter- 
posed between a membranous bag, or synovial membrane; so 
called because it contains synovia, or joint-oil—an arrangement 
which facilitates movement and diminishes friction. Adjacent 
vertebree may completely unite with each other or with other 
parts of the skeleton. Such parts are then said to be anchylosed 
together. 
The vertebra are divisible into four categories :—(1) Those 
of the neck, or cervical vertebre; (2) those of the trunk, or 
dorsal vertebre ; (3) those intermediate between these and the 
