Next explore lateral flexion at the same articulations. Keeping your ad balanced upright, slowly tilt your forehead to the right and your chin t 'he left, and then repeat in the opposite direction, envisioning the ax of rotation extending between the mouth and the back of the head. A, iin. don't bend the rest of the neck. To the extent that you can feel the chii ind the forehead move in opposite directions, you will be feeling the side-to ide slipping of the cranium on the upper surface of the atlas. If you concent ite on carrying this particular movement to its maximum, it feels extensi e at least 15' in each direction. Again, for contrast, as soon as you 've reached the limit of movement between the atlas and the head, flex he head and neck as far as possible laterally. This gives you a different sensai on, like the movement of the head in a Jack-in-the-Box.
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the movement of the atlas around the axis
Now we can return to our more immediate interest in twisting as we explore the rotation of the atlas (plus the skull) around the odontoid process of the axis (figs. 4.8 and 7.2). Do the following experiment. Sit quietly with the spine and head upright. Without dipping the head either forward, backward, or to the side, slowly turn it to one side and then the other through a total excursion of 60-90°. In other words turn 30-45° to the right and 30-45" to the left. Try to relax. Turn slowly, then a little more quickly, then again slowly This is the movement of the atlas around the odontoid process. You can envision it if you make a circle with the thumb and index finger of one hand, insert the thumb of the other hand (representing the odontoid process) against the back of the circle, and rotate the circle through an excursion of 6o°, in other words 30° each way.
Next, for contrast, turn the head as far as possible in each direction. If you have fairly good flexibility you can turn about 90° each way. The first portion of the twist is 30-45° of rotation between the atlas and the axis, and the remainder of the 45-60° represents twist in the rest of the neck.
To return to the delicate twisting that takes place only between the atlas and the axis, notice what happens as you move past the symmetric central balancing position with the head facing straight ahead. Something takes place—a change of speed, first a slowing down, then a speeding up as you pass center, a movement so subtle that you will not feel it unless you are relaxed. What is happening is that a cam action is lilting your head slightly as you pass center. It's as if a motorized toy car were approaching a little hill. You can feel your head rise as you approach the hill, and a gathering resistance that peaks at the top and then diminishes as you cross over. Find the little hill, park exactly on top, and then move slightly to one side and then the other. If you watch carefully you will notice that the high point is not perfectly on line in the midsagittal plane of the body but is usually somewhat to the left or right of center, and that you habitually adjust your body posture so that your head rests on the side that keeps you facing relatively straight to the front.
the relationship between the axis and the atlas
To understand why this happens we need to look more carefully at the relationship between the axis and the atlas. As we discussed in chapter 4, each individual vertebra is composed of a vertebral body and a vertebral fcrch. The exception to this is the atlas, which is simply a ring of bone (figs. 7-1-2). In the embryo what had originally been the body of the atlas became
L'ncorporated into the axis, bestowing on the axis the equivalent of two fijsed-toget her vertebral bodies, the top portion of which is the odontoid
this process that the atlas rotates, and it is from this function that the ax-was named (figs. 4.8 and 7.2).
The reason you feel the rising-on-center sensation as you twist is th the lateral joint surfaces between Ci and C2 are not in perfect appositi« i) unless the head is turned slightly to one side or the other. According rotation off that center is accompanied by a slight vertical descent of t e head; keeping the head directly on the high point once you have locate! t is an interesting exercise in balancing and concentration. Enthusiasts <r working with right-left balance might suggest that shifting the head to ie side on which it does not usually rest will gradually bring the rising-on-cet t sensation into the midsagittal plane of the body and make the joint ni e symmetrical. It's an interesting idea, although speculative.
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