54 THE ENGINES OF THE HUMAN BODY 



if we roll our heads backwards to such an extent that the 

 bony edge of the opening in the base of the skull is made 

 to press hard against the brain stem and crush it ? That, 

 of course, would mean instant death. Such an accident 

 has been made impossible (i) by making the opening in 

 the base of the skull so much larger than the brain stem 

 that in extreme movements there can be no scissors-like 

 action ; (2) the muscles which move the head on the 

 atlas arrest all movements long before the danger-point is 

 reached ; (3) even if the muscles are caught off their 

 guard, as they sometimes are, certain strong ligaments — 

 fastenings of tough fibres — are so set as to automatically 

 jam the joint before the edge of the foramen can come in 

 contact with the brain stem. 



These are only some of the devices which Nature had 

 to contrive in order to secure a safe passageway for the 

 brain stem. But in obtaining safety for the brain stem 

 the movements of the head on the atlas had to be limited 

 to mere nodding or side-to-side bending. The move- 

 ments which are so necessary to us, that of turning our 

 heads so that we can sweep our eyes along the whole 

 stretch of the skyline from right to left, and from left to 

 right, were rendered impossible. This defect was also 

 overcome in a simple manner. The joints between the 

 first and second vertebrae — the atlas and axis — were so 

 modified that a turning movement could take place be- 

 tween them instead of between the atlas and skull. 

 When we turn or rotate our heads the atlas, carrying the 

 skull upon it, swings or turns on the axis. When we 

 search for the manner in which this has been accom- 

 plished, we see again that Nature has made use of the 

 simplest means at her disposal. When we examine a 

 vertebra in the course of construction within an unborn 

 animal, we see that it is really made up by the union of 

 four parts (see fig. 12) : a central block which becomes 

 the " body " or supporting part ; a right and a left arch 

 which enclose a passage for the spinal cord ; and, lastly, 

 a fourth part in front of the central block which only 

 becomes big and strong in the first vertebra— the atlas. 



