THE THORACIC VERTEBRAE 



37 



ventrally with that of the dens. The posterior extremity has the usual cavity. 

 The ventral spine resembles that of the typical vertebrae. 



The arch presents in the young subject a notch on each side of its anterior 

 border; this is converted into a foramen by a ligament which ossifies later. The 

 posterior border has the usual notches. 



The posterior articular processes are typical. ■ 



The transverse processes are small, single, and project backward. The 

 foramen transversarium is small. 



The spinous process is very large and strong. Its free border is rough, thickens 

 posteriorly, and is continued to the articular processes by two ridges. The lateral 

 surfaces are concave and rough for muscular attachment. 



Development. — The axis has six or seven centers of ossification. In addition 

 to the usual five, one or two appear for the dens, which is regarded as the displaced 

 body of the atlas. A nucleus behind the dens, which remains distinct to three or 

 four years of age, is considered to be the head of the axis. 



THE THORACIC VERTEBILE 



The thoracic vertebrae (Vertebrae thoracales) are usually eighteen in number in 

 the horse, but there are sometimes nineteen, rarely seventeen. As regional char- 

 acters we note the surfaces for articulation with the ribs and the length and form of 

 the spinous processes. Those in the 

 middle of the series are the most 

 typical and present the following fea- 

 tures: 



1. The bodies are short and con- 

 stricted in the middle. The ends are 

 expanded and have articular surfaces 

 which are not strongly curved; the an- 

 terior surface is convex, the posterior 

 concave. On the upper part of each 

 side are anterior and posterior costal 

 facets (Fovea costalis cranialis, cau- 

 dalis), which, with those of adjacent 

 vertebrae and the intervening fibro-car- 

 tilages, form sockets for the heads of 

 the ribs. 



2. The arches are small. Their 

 posterior notches are relatively large 

 and are often converted into foram- 



Spinous 

 process 



Articular 

 process 



are 



Facet for tubercle 

 of rib 



Facet for 

 head 

 of rib 



Body 



Fig. 13. — Seventh Thoracic Vertebra of Horse, An- 

 terior View. 



3. The articular processes 



small. The anterior processes are in 



fact represented only by two oval facets 



on the anterior part of the arch which 



face almost directly upward. Thepos- ^, . . , . 



terior processes spring from the base of the spinous process; their facets face 



almost directly downward. ^ ^, r j 



4 The transverse processes are short, thick, and tuberous at the tree end. 

 Each has a facet (Fovea transversaria) for articulation with the tubercle of the 

 rib which has the same serial number. , , , i 



5 The spinous process is large, narrow, and slopes upward and backward. 

 The anterior border is thin, the posterior wider and furrowed. The summit is 

 expanded and rough. 



