26 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 9. — The tenth to the twelfth thoracic and the first and second lumbar vertebras seen from the side 

 and slightly from behind (J). 



Fig. 10. — The tenth thoracic vertebra from above (y). 



Fig. 1 1 . — The sixth thoracic vertebra from the side ( T ) . 



Fig. 12. — The third lumbar vertebra seen from above ( T ). 



epistrophei) and projects into the anterior compartment of the spinal foramen of the atlas. This 

 odontoid process is cone-shaped with a rounded apex, and presents an anterior facet for articu- 

 lation with the anterior arch of the atlas and a posterior articular facet which is not always dis- 

 tinct. Instead of articular processes the upper part of the body presents corresponding articular 

 surfaces for connection with the atlas; these surfaces are slightly convex and but a trifle inclined 

 from the horizontal. 



The spinous process (Fig. 4) is fairly well developed and always distinctly bifid; the trans- 

 verse processes, on the contrary, are smaller than those of the atlas and their tubercles and sulci 

 nervi spinalis are likewise absent, although there is usually a shallow groove for the second 

 spinal nerve on each side behind the superior articular facet. The inferior articular processes 

 are more inclined than the superior ones, and already show the characteristics of those of the 

 flexion vertebrae. 



The foramina transversaria of the articulated cervical vertebra form a canal for the passage of the vertebral artery 

 and vein (the former runs through the upper six only). The markedly developed anterior tubercle of the transverse pro- 

 cess of the sixth cervical vertebra is known as the carotid tubercle or tubercle of Chassaignac {tuberculum caroticum). 



THE THORACIC VERTEBRAE. 



On account of the independent development of the ribs in the thoracic region, we find no 

 fused costal rudiments in the thoracic vertebrae such as occur throughout the remainder of the 

 true vertebrae, and the thoracic vertebrae are consequently of the purest type. 



Their bodies (Figs. 9, 10, 11, and 21) increase in size and height from above downward. 

 In the upper thoracic vertebras their surfaces arc elliptical, like those of the cervical region, 

 but as we pass downward they become rounder, then cordiform, and finally reniform in the 

 lower members of the series, which approximate the form of the lumbar vertebrae. And not 

 only do the bodies become larger, but their lateral diameter especially is increased as they gradu- 

 ally approach the form of the lumbar vertebrae. Their contiguous surfaces are almost per- 

 fectly flat. The spinal foramen (Fig. 10) is not only absolutely, but even relatively smaller 

 than that of the cervical vertebrae, and in the upper members of the series it is rounded, while 

 in the lower ones it is rather triangular. The upper and lower margins of the bodies each present, 

 immediately in front of the pedicles, a demifacct for the head of a rib (Fig. 11). The first and 

 the two (or three) lowest vertebrae, however, show deviations from this arrangement, the former 

 having an entire facet upon the upper margin, and each of the latter (Fig. 9) presenting an entire 

 facet toward the middle of the body of the vertebra. 



The articular surfaces for the heads of the ribs are placed on two adjacent thoracic vertebrae in such a manner 

 that each vertebral margin does not receive exactly one-half of the articulation, but toward the lower end of the series 

 more than half and then two-thirds of the entire surface occurs upon the lower vertebra until the eleventh, and occasion- 



