THE ATLAS 



33 



articular processes are shorter, thicker and further apart; they are connected with 

 the anterior ones by a thick ridge. The spinous process is less rudimentary; it 

 is half an inch or more (ca. 1.5 cm.) in height. The transverse processes have 

 three branches; the third part is a thick, almost sagittal plate, which forms with 

 its fellow and the body a wide ventral groove on the posterior part of the vertebra; 

 the other branches correspond to those of the typical vertebra?, but are short and 

 thicker. The foramen transversarium is large; below its posterior end is a fossa. 

 The ventral crest is small and is less prominent posteriorly. 



The third branch of the transverse process and the fossa are sometimes absent on one side. 



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 demifacet on either 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 transversarium. The ventral crest is replaced 

 by a pair of tubercles. 



In some specimens a large foramen transversarium is present on one side or (rarely) on 



both. 



Dorsal arch 



A nterior 



articular 



cavity 



Intervertebral 

 foramen 



Vertebral 

 foramen 



Foramen 

 transver- -- 

 sarium Ventral 

 arch 



Fossa 

 atlanlis 



Fig. 9. — Atlas of Horse, Anterior View. 

 (After Schmaltz, Atlas d. Anat. d. Pferdes.) 



Ventral tubercle 



Fig. 10. — Atl.^s of Horse, Posterior View. 

 Schmaltz, Atlas d. Anat. d. Pferdes.) 



(After 



The Atlas 



This vertebra is decidedly atypical 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 consists of two lateral masses connected by dorsal and ventral arches. 



The lateral masses (Massse laterales) present two deep oval anterior articular 

 cavities (Fovese articulares craniales) which receive the occipital condyles; thej^ are 

 separated by a wide notch above and a narrow one below. The outer margin is also 

 notched, and a triangular non-articular depression cuts into the inner part of each 

 cavity. The posterior articular surfaces (Facies articulares caudales) are some- 

 what saddle-shaped ; they are confluent on the ventral arch below, but are widely 

 separated above, and do not conform in shape to the corresponding surfaces of the 

 axis. 



The dorsal arch (Arcus dorsalis) presents a median dorsal tubercle (Tubercu- 

 lum dorsale) and is concave below. It is perforated on either side near its anterior 

 margin by the intervertebral foramen. The anterior border is deeply notched, and 

 the posterior is thin and concave. 



The ventral arch (Arcus ventralis) is thicker, narrower, and less curved than 

 3 



