THE THORACIC VERTEBRE 37 
ventrally with that of the dens. The posterior extremity has the usual cavity. 
The ventral spine resembles that of the typical vertebre. 
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 VERTEBRZ 
The thoracic vertebre (Vertebre 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 
Spinous 
concave. On the upper part of each eects 
side are anterior and posterior costal 
facets (Fovea costalis cranialis, cau- 
dalis), which, with those of adjacent 
vertebre and the intervening fibro-car- 
tilages, form sockets for the heads of Articular 
ee process | Transverse 
; - process 
2. The arches are small. Their Facet for tubercle @ 
. : fry : 
posterior notches are relatively large of rib 
and are often converted into foram- Facet for 
ina. head 
3. The articular processes are cha 
small. The anterior processes are in Body 
fact represented only by two oval facets Fic. 13.—SrvenTH THoRacIc VERTEBRA OF Horse, AN- 
on the anterior part of the arch which TeRioRr VIEW. 
face almost directly upward. The pos- 
terior processes spring from the base of the spinous process; their facets face 
almost directly downward. 
4. The transverse processes are short, thick, and tuberous at the free end. 
Each has a facet (Fovea transversaria) for articulation with the tubercle of the 
rib which has the same serial number. 
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. 
