918 THE OSTEOLOGY AND MYOLOGY OF THE DEATH ADDER, 



being caused by the lateral portions projecting more forward. 

 This has reference to the fact that the vertebrae generally move in 

 the transverse and not in the vertical plane. The " ball" of the 

 posterior articular surface is almost a hemisphere. On looking at 

 the surface in profile the curve of the upper third of the ball is 

 seen to be the circumference of a smaller circle than the lower 

 two-thirds, while the lateral portions of the rim are produced 

 forward so as to correspond with the lateral edges of the socket. 



The centrum as a whole is somewhat pyramidal, .the base being 

 posterior. The base slopes from above downwards and backwards, 

 and it is upon this surface that the "ball" of the posterior articular 

 surface rests. It follows from this that the axis of the posterior 

 face, instead of corresponding with the axis of the centrum, makes 

 an angle of about thirty degrees with the long axis of the body. 

 The advantage of this will be seen hereafter. 



The anterior portion of the centrum corresponding to the 

 apex of the pyramid bears the concave anterior articular face. 

 The dorso-ventral axis of this face is inclined from before, 

 downwards, and backwards. Owing to this the lower three-fifths 

 of the socket rests on the front of the pyramid, while the upper 

 two-fifths is free, and inclines forward. The reason for this 

 becomes evident when we take a longitudinal vertical section of 

 two vertebrae ; we then see that the superior part of the cup rests 

 on the upper and most curved portion of the ball, while the lower 

 three-fifths rests on the less curved portion of the ball. We thus 

 have a ball and socket joint, the dorso-ventral axis of which 

 is downwards and backwards, and thus is formed a joint capable 

 of withstanding greater force from above downwards than if the 

 axis of the ball and socket was parallel to the long axis of the 

 body. 



From the anterior portion of the external surface the diapo- 

 physis and parapophysis spring ; while from the inferior surface a 

 strong recurved hypapophysis projects. On each side of the base 

 of the hypapophysis are seen two excavations from which the 

 levatores costarum interni spring. The superior surface forms the 

 floor of the neural canal. 



