100 NATURAL THEOLOGY. 



[Tliis figure represents the uppermost vertebra, or atlas ; and 

 the condyles, mentioned in the former figure, sink into the articu- 

 lating surfaces of this vertebra, permitting the nodding motions. 

 a and b are the articulating surfaces ; c is a surface which receives 

 the tooth of the vertebra below; d the circle through which the 

 spinal marrow passes.] 



not between the head and the uppermost bone of 

 the neck, where the hinge is but between that 

 bone and the bone next underneath it. It is a 

 mechanism resembhng a tenon and jnortise. This 

 second, or uppermost bone but one, has what ana- 

 tomists call a process, viz., a projection, somewhat 

 similar, in size and shape, to a tooth; which tooth 

 entering a corresponding hole or socket in the 

 bone above it, forms a pivot or axle, upon 

 which that upper bone, together with the head 

 which it supports, turns freely in a circle ; and as 

 far in the circle as the attached muscles permit 

 the head to turn. Thus are both motions perfect 

 without interfering with each other. When we 

 nod the head, we use the hinge-joint, which lies 

 between the head and the neck. When we 

 turn the head round, we use the tenon and mor- 

 tise, which runs between the first bone of the neck 

 and the second. ^^ 



** The meaning of our author is obvious here ; but the tenon 



