44 



ELEMENTS OF MAMMALIAN ANATOMY. 



intervertebral foramen for the exit of a spinal nerve; (6) 

 the articular processes, two of which project craniad and 

 two caudad from each vertebra. 

 The former are called the anterior 

 articular processes and the latter 

 posterior articular processes. 



The first . cervical vertebra, or 

 atlas, is characterized by its large 

 horizontally expanded transverse 

 processes, the absence of a spinous 

 FIG. 22. PLAN OF A VER- process, and its rudimentary body. 

 Its true body in the embryo becomes 



sp, Spinous process ; ar, J J 



articular process; ns, united to the axis as the odontoid 

 transverse process ; nc, <- - > r , i 



neural canal; pd, pedi- process. The cranial margin of the 

 cle; Im, lamina; en, neural arch is prolonged at each 



centrum or body. . 



side into a process or articulation 



with the occipital condyles (Fig. 17) of the skull. The 

 root of this articular process is pierced by a foramen giving 

 passage to the first spinal nerve 

 and the vertebral artery and vein 

 (Fig. 23, fr). From the lateral 

 opening of this foramen (fr) a 

 groove is continued ventrad to 

 the middle of the transverse proc- 

 ess, where it leads into the trans- 

 verse foramen common to the 

 first six cervical vertebrae. The 

 vertebral artery and vein course 

 through this canal (Fig. 23). 



The axis, or second vertebra, 

 is characterized by its odontoid 

 process, which projects within the 

 atlas, and also by its elongated spinous process, which pro- 

 jects both craniad and caudad. The transverse process pro- 



23. DORSOCAUDAL As- 



PECT OF ATLAS. 



tr, Transverse process ; nc, 

 neural canal; sp, spinous 

 process; fr, aperture of 

 the transverse foramen 

 which enters the atlas at 

 vf\ azg, anterior artic- 

 ular process ; zg, poster- 

 ior articular process. 



