52 THE ENGINES OF THE HUMAN BODY 



which fills the whole chamber of the skull, to the spinal 

 cord situated in the canal of the backbone. By means of 

 these delicate fibres the brain dispatches messages which 

 control the muscular engines of the limbs and trunk. 

 Through it, too, ascend countless fibres along which 

 messages pass from the limbs and trunk to the brain. 

 In creating a movable joint for the head, then, a safe- 

 passage had to be obtained for the medulla — that part of 

 the great nerve stem which joins the brain to the spinal 

 cord. The medulla is part of the brain stem. 



This was only one of the difficulties which had to be 

 overcome. The eyes are set on the pre-fulcral lever of 

 the head. For our safety we must be able to look in 

 all directions — over this shoulder or that. We must 

 also be able to turn our heads so that our ears may 

 discover in which direction a sound is reaching us. In 

 fashioning a fulcral joint for the head, then, two different 

 objects had to be secured : free mobility for the head, and 

 a safe transit for the medullary part of the brain stem. 

 How well these objects have been attained is known 

 to all of us, for we can move our heads in the freest 

 manner and suffer no damage whatsoever. Indeed, so 

 strong and perfect is the joint, that damage to it is one of 

 the most uncommon accidents of life. 



Let us look, then, how this triumph in engineering 

 has been secured. In her inventive moods Nature 

 always hits on the simplest plan possible. In this case 

 she adopted a ball-and-socket joint — the kind by which 

 older astronomers mounted their telescopes. By such 

 a joint the telescope becomes, just as the head is, a 

 lever of the first order. The eyeglass is placed at one 

 end of the lever, while the object-glass, which can 

 be swept across the face of the heavens, is placed at the 

 other or more distant end. In the human body the first 

 vertebra of the backbone — the atlas — is trimmed to form 

 a socket, while an adjacent part of the base of the skull 

 is shaped to play the part of ball. Having thus hit upon 

 the kind of joint to be used, the next point was to secure 

 a safe passage for the brain stem. That, too, was worked 



