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 

 transversariiim, 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 bj^ 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 vertebrae, but are short and thicker. ^ 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. ^ The ventral crest is re- 

 placed by a pair of tubercles. 



The Atlas 



This vertebra is decidedly atypi- 

 cal 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 a 

 very large vertebral foramen, and con- 

 sists of two lateral masses connected 

 by dorsal and ventral arches. 



The lateral masses (Massse later- 

 ales) present two deep oval anterior 

 articular cavities (Foveae articulares 

 craniales) 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. 



1 The third branch of the transverse process and the fossa are sometimes absent or reduced 

 on one side. . , , 



2 In some specimens a large foramen transversarimn is present on one side or (rarelyj on 

 both sides. 



Fig. 11. — Atlas of Horse, Dorsal View after Removal 

 OF Dorsal Arch. 

 1, Anterior articular cavities; S, 2', posterior articular 

 surfaces; 3, articular surface of ventral arcli for dens of 

 axis; 4, transverse ridge; B, B' , alar foramina; 6, 6', foram- 

 ina transversaria. 



