44 THE CAT. [CHAP. in. 



10. The first vertebra of all, called the ATLAS, also differs con- 

 spicuously from every other bone of the spine. It differs by the 

 large size of the aperture it encloses and by the smallness of the part 

 which, at first sight, appears to represent the centrum, as also by 

 the absence of a neural spine. The ring it forms is wider above 

 than below, and 'it is this wider part which corresponds to the 

 neural arches of the other vertebrae. The narrower ventral part 

 receives upon it the odontoid process of the axis. The part which 

 seems to, but does not truly represent the centrum, articulates by 

 its upper surface with the under surface of the odontoid process, 

 while its own under surface developes a slight median prominence (y) 

 or hypapophysial tubercle. The neural arch (n) is slightly wider 

 from behind forwards than in the other cervical vertebra), and there 

 is no neural spine, or only a minute rudiment of it (). Each 

 neural lamina is perforated just above the anterior articular surface 

 (Fig. 21, B, /). The vertebral artery and sub-occipital nerve 

 traverse this foramen. The sides of the atlas vertebra are termed 

 " the lateral masses," and give rise to the great, wing-like, trans- 

 verse processes (tf), and to large anterior and posterior articular sur- 

 faces. There are no true zygapophyses. The transverse processes are 

 longer and larger than in the other cervical vertebrae, and consist 

 of a tubercular and capitular process, united at their distal ends, 

 or enclosing a small foramen for the vertebral artery (Fig. 21, 

 C and D, /), and then expanding into a plate of bone, one side of 

 which looks upwards and the other downwards. Of the four large 

 articular surfaces two are situated behind the root of each transverse 

 process (Fig. 21, C, z), and correspond in position with the anterior 

 articular surfaces of the axis, before described. The anterior pair 

 of surfaces (Fig. 21, B and E, s) are very large, cup-shaped, and oval, 

 converging interiorly, and looking inwards as well as forwards. 

 They receive and articulate with two prominences of the hinder 

 end of the skull. At the inner margin of each is a minute, smooth, 

 rounded tubercle. The hinder pair of articular surfaces (Fig. 21, 

 C and D, z) are smaller than the anterior pair, and are more elongated 

 in shape and flatter. They are inclined a little inwards as well as 

 upwards and backwards, and join the anterior articular surfaces of 

 the axis vertebra. 



The atlas is formed to turn on the odontoid process of the axis as 

 on a pivot, as will be further explained when the ligaments come 

 to be described. 



11. Next to be noticed are the three (sometimes four) vertebra) 

 which are often called "false," because they anchylose together into 

 one bony mass, and so constitute the SACRUM. 



This bone immediately succeeds the hindmost lumbar vertebra, 

 and is roughly quadrangular in form, but the transverse diameter of 

 its hinder end is considerably less than that of its anterior portion. 



The composite nature of the sacrum is plainly manifested (in the 

 fully ossified bone of even the most aged individuals) by its processes 

 and perforations, and by the transverse markings of its ventral surface. 



