240 THE VERTEBRATE SKELETON. 



It has a short cylindrical centrum deeply concave in front 

 and convex behind. From the anterior part of the ventral 

 surface of the centrum arises a short hypapophysis, and 

 on each side is a facet with which the lower limb (capi- 

 tulum) of the cervical rib articulates. The neural arch is 

 strongly developed and drawn out dorsally into a long neural 

 spine, in front of which are a pair of upstanding processes 

 bearing the prominent upwardly and inwardly directed pre- 

 zygapophyses. At the sides and slightly behind the neural 

 spine are a corresponding pair of processes bearing the post- 

 zygapophyses, which look downwards and outwards. At 

 the point where it joins the centrum the neural arch is drawn 

 out into a short blunt transverse process with which the 

 upper limb (tuberculum) of the cervical rib articulates. The 

 sides of the neural arch are slightly notched behind for the exit 

 of the spinal nerves. 



The first or atlas vertebra differs much from any of the 

 others, and consists of four quite detached portions, a ventral 

 arch, with two lateral portions and one dorsal. The ventral 

 arch (fig. 41, 4) is flat below and slightly concave in front, 

 forming together with two flattened surfaces on the lateral 

 portions a large articulating surface for the occipital condyle 

 of the skull. Its posterior face is bevelled off and forms with 

 a second pair of facets on the lateral portions a surface with 

 which the odontoid process of the second vertebra articulates. 

 The postero-lateral surfaces of the ventral arch also bear a pair of 

 little facets with which the cervical ribs articulate. The lateral 

 portions are somewhat flattened and expanded, and bear in ad- 

 dition to those previously mentioned a pair of small downwardly 

 directed facets, the postzygapophyses, which articulate with the 

 prezygapophyses of the second vertebra. The dorsal portion 

 (fig. 41, 1) is somewhat triangular in shape, and overhangs the 

 occipital condyle. It is often regarded as the neural arch of a 

 vertebra in front of the atlas and is called the pro-atlas ; but 

 as it is a membrane bone it is not properly a vertebral element. 



