THE ATLAS 35 
the articular processes of the same side on the fourth and fifth; on the third it does 
not reach the anterior process. 
4. The transverse processes are large and plate-like. Each arises by two 
roots, one from the arch and one from the body; between these is the foramen 
transversarium, through which the vertebral vessels and a nerve pass. The aggre- 
gate of these foramina constitutes the canalis transversarius. The process divides 
laterally into anterior and posterior branches, which are thickened and rough for 
muscular attachment. 
5. The spinous process has the form of a low crest (Crista spinosa), which 
widens behind, and is connected by ridges with the posterior articular processes. 
The sixth cervical vertebra has the following distinctive features: It is shorter 
and wider than the fifth. The arch is large, especially posteriorly. The posterior 
articular processes are shorter, thicker, and further apart; each is connected with 
the corresponding anterior one by a thick ridge. The spinous process is less rudi- 
mentary; it is half an inch or more (ca. 1.5 cm.) in height. The transverse proc- 
esses have three branches; the third part is a thick, almost sagittal plate, which 
forms with its fellow and the body a wide ventral groove; the other branches 
correspond to those of the typical vertebre, but are short and thicker.t The fora- 
men transversarium is large; below its posterior end there is a fossa. The 
ventral spine is small and is less prominent posteriorly. 
The seventh cervical vertebra is readily distinguished by the following charac- 
ters: It is shorter and wider than the others. The body is flattened dorso-ventrally 
and wide, especially behind; here it has a facet on each side for articulation with 
part of the head of the first rib. The arch and its notches are large. The anterior 
articular processes are wider and longer than the posterior pair. The spinous 
process is an inch or more (ca. 3 cm.) 
in height. The transverse process is 
undivided, and has no foramen trans- 
versarium.2 The ventral crest is re- 
placed by a pair of tubercles. 
THE ATLAS 
This vertebra is decidedly atypi- 
eal in form and structure. The body 
and spinous process are absent. It 
has the form of a strong ring, from 
which two curved plates, the wings, 
project laterally. The ring incloses ¢ 
very large vertebral foramen, and con- 
sists of two lateral masses connected 
Fie. 11.—Arttas oF Horse, Dorsat VIEW AFTER REMOVAL 
or Dorsat ARCH. 
1, Anterior articular cavities; 2, 2’, posterior articular 
by dorsal and ventral arches. 
The lateral masses (Masse later- 
ales) present two deep oval anterior 
surfaces; 3, articular surface of ventral arch for dens of 
axis; 4, transverse ridge; 4, 5’, alar foramina; 6, 6’, foram- 
ina transversaria. 
articular cavities (Fovexe articulares 
eraniales) which receive the occipital condyles; they are separated by a wide notch 
above and a narrow one below. The lateral margin is also notched, and a trian- 
gular non-articular depression cuts into the medial part of each cavity. The pos- 
terior articular surfaces (Facies articulares caudales) are somewhat saddle-shaped; 
they are confluent on the ventral arch, but are widely separated dorsally, and do 
not conform in shape to the corresponding surfaces of the axis. 
‘The third branch of the transverse process and the fossa are sometimes absent or reduced 
on one side. 
ee, ie some specimens a large foramen transversarium is present on one side or (rarely) on 
oth sides. 
