A Book of Exposition | Page 4

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 see, 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. The kind of joint to be used having been hit upon, the next point was to secure a safe passage for the brain stem. That, too, was worked 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. 3, A), covered by smooth slippery 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 is two hollows on the upper surface of the atlas into which the occipital condyles fit (Fig. 3, B). Merely two parts of the brim of the cup have been preserved to provide a socket for the condyles or ball.
[Illustration: Fig. 3.--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.]
As we bend our heads, the occipital condyles revolve or glide on the sockets of the atlas. But what will happen 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 (1) 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 automatically to 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 movements 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 between 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 accomplished, 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