VERTEBRAL COLUMN 119 



THE ATLAS VARIATIONS IN DEVELOPMENT. 



The cephalic margin of the neural arch may be straight or gently 

 emarginate, with a single median tubercle or with two strong tubercles. 

 The caudal margin, dorsal to the caudal articular processes, may be 

 incised or straight. The arterial foramen on each side of the cephalic 

 end of the laminse may be merely a notch in the cephalic margin, 

 owing to the failure of the cephalo-medial wall to ossify. The 

 foramen may be double. This appears to be a variation comparatively 

 rare, inasmuch as it was found only once, and on the right side, in an 

 examination of seventy-five specimens. 



A trace of a neural spine as a cephalic dorsal tubercle has been 

 observed in three out of fifty specimens. 



The dorsal surface of the body, just caudal to the cephalic edge, is 

 sometimes marked by a transverse ridge on the line of attachment of 

 a ligament. The longus colli tubercle on the caudal margin may 

 be absent. 



The cephalic margin of the cephalic articular processes may 

 project beyond the neural arch or may be almost on a line with it. 

 Their articular surfaces may continue on the cephalic margin of the 

 body. In some specimens the caudal articular surfaces encroach upon 

 the edge of the body. 



The edge of the transverse process is sometimes prolonged and 

 united with the cephalic articular process, converting the arterial 

 groove into an additional lateral foramen. The edge is subject to much 

 variation in the degree of curvature. The caudal margin may be sepa- 

 rated from the caudal articular surface by a deep notch, or the two may 

 be continuous. In some cases an oblique ridge on the ventral surface, 

 caudal to the area of origin of the levator clavicular, separates the 

 caudal articular process from the transverse process. 



The cephalic opening of the arterial canal may be cephalic or 

 caudal to the middle of the cephalo-caudal diameter of the root of 

 the transverse process. The arterial groove cephalic to the foramen 

 is shallow or deep, and occasionally is converted into a cleft by the 

 turning ventrally of that part of the transverse process. One of the 

 caudal arterial foramina may be larger than the other. 



