OUR BONES ARE LIVING LEVERS 



53 



out in the simplest fashion. The central parts of both 

 ball and socket were cut away, or, to state the matter more 

 exactly, were never formed. Thus a passage was obtained 

 right through the centre of the fulcral joint of the head. 

 The centre of the joint was selected because when a lever 

 is set in motion the part at the fulcrum moves least, and 

 the medulla, being placed at that point, is least exposed to 

 disturbance when we bend our heads backwards, forwards, 

 or from side to side. When we examine the base of the 

 skull all that we see of the ball of the joint are two 

 knuckles of bone (fig. 1 1, A), covered by smooth slippery 



OCCIPITAL CONDYLES 

 (Ball) 



ARTICULAR FACETS 



(Socket) 



p- yt Spinal cord 

 spinal canal^ 



Fig. ii. — A, The opening in the base of the skull, by which the brain stem passes 

 to the spinal canal. The two occipital condyles represent part of the ball 

 which fits into the cup formed by the atlas. B, The parts of the socket on 

 the ring of the atlas. 



cartilage or gristle, to which anatomists give the name of 

 occipital condyles. If we were to try to complete the 

 ball, of which they form a part, we should close up the 

 great opening — the foramen magnum — which provides a 

 passageway for the brain stem on its way to the spinal 

 canal. All that is to be seen of the socket or cup are two 

 hollows on the upper surface of the atlas into which the 

 occipital condyles fit (fig. II, B). Merely two parts of 

 the brim of the cup have been preserved to provide a 

 socket for the condyles or ball. 



As we bend our heads the occipital condyles revolve or 

 glide on the sockets of the atlas. But what will happen 



