I20 



HUMAN ANATOMY. 



the arrangement is that the part which in the ordinary process of development 

 should become the body of the atlas is instead fused with the body of the axis. 



The atlas, having no body, consists of two lateral masses, connected by a short 

 anterior arch and a long posterior one. The lateral masses present the articular 

 facets on their lower and upper surfaces. The inferior look downward and slightly 

 inward, and are very slightly concave from side to side. The superior facets are 

 oval concavities the backs of which are strongly raised from the surface. Their 

 long axis runs forward and inward, the outer wall being decidedly higher than the 

 inner. The articular facet narrows at the middle, and is often marked by a trans- 

 verse ridge at this point ; rarely it is divided into two parts. The articular surfaces 

 of the two sides sometimes very nearly correspond with parts of the surface of a 

 single imaginary sphere. 



Their variation in all respects is great. Thus, Macalister ^ finds in one hundred 

 bones that the distance between the front ends of the two facets varies from ten to 

 twenty millimetres, being usually from fifteen to twenty millimetres, and that the 

 hind ends are from thirty-two to fifty millimetres apart, the greater number being 

 separated from thirty-five to forty millimetres. The angle formed by the intersec- 



FiG. 149. 



Posterior tubercle 



Posterior arch 



Transverse foramen 



Anterior a 



Inferior articular facet 



Position of transverse ligament (dotted lines) 



Facet for odontoid process Anterior tubercle 



The atlas from below. 



tion of the prolonged axes of the articular facets ranges from thirty-two to sixty- 

 three degrees. Each lateral mass presents a rough tubercle on the inner side between 

 which passes the transverse ligament holding the. odontoid process close against the 

 anterior arch. The anterior arch is compressed from before backward. It presents 

 the anterior tubercle in front in the median line, and behind has a slightly concave 

 articular facet for the odontoid process. The posterior arch bounds the spinal canal 

 behind. The transverse ligament, confining the odontoid process, bounds the spinal 

 canal in front, and, being in place, the transverse diameter of the canal is the longer. 

 The place of the spinous process is taken by the posterior tubercle. The transverse 

 processes extend farther out than any in the cervical region. Each ends in a single 

 flattened knob with a surface slanting downward and forward. Bifurcation is rare. 

 The transverse foramen is at its base ; from the foramen a groove for the vertebral 

 artery crosses the root of the posterior arch and winds round behind the raised 

 border of the articular surface. This groove is occasionally bridged over by a little 

 arch of bone extending from the edge of the articular surface either to the trans- 

 verse process or to the posterior arch. 



Variations. — The atlas may be fused with the occipital bone in various ways; 

 this may occur by the pathological destruction of the joint, or the arch, or a 



* Journal of Anatomy and Physiology, vol. xxvii., 1893. 



