total change is of the order of 5-7 cm, and is greater on the posterior than the anterior aspects. Similarly, on lateral flexion, the canal is lengthened on the convex side and shortened on the concave."

b) "At its cranial extremity the spinal dura mater is attached to the circumference of the foramen magnum, and at its caudal end it is anchored to the coccyx by the filum terminale. When the trunk is fully flexed the dura is under tension, as is also the cord, and stretching occurs. Part of this tension is transmitted from the dura mater via the dentate ligaments to the pia mater, but by far the larger component of the tension is set up directly in the cord by virtue of its anchorage at its two extremities, namely the brain stem and the cauda equina. From the biomechanical aspect the spinal cord therefore cannot be considered in isolation but must be treated as a continuous tract of nervous and supporting tissues, from the mesenchepalon to the conus medullaris ... referred to as the pons-cord tissue tract, or simply the pons-cord tract."

c) "The static and dynamic properties of the pons-cord tract constitute a self-contained biomechanical compartment. In extension of the spine from the neutral posture, the axis of the spinal canal, and hence that of the tract, is shortened and the tissue slackens and folds. When the neutral posture is assumed the tract recovers its original length, the slack is taken up and the folds are eliminated. In flexion, in which the length of the canal is increased, the tract is stretched elastically. During these spinal movements the axons and blood-vessels of the spinal cord undergo deformation similar to that of the cord as a whole."

d) "Concerning pathological sources of deformation within the canal, it has been traditional to think solely in terms of compression of the spinal cord or nerve roots. From the work reported earlier and from the new evidence presented here, it is abundantly clear that this concept is too narrow. Even a demonstrably compressive force generates short-range axial tension. In a compressive brain or spinal cord injury the nerve fibers and blood vessels are not compacted, but stretched and torn apart. A typical situation in which local tension is set up in the cord is that where, during extension of the spine, it is pinched or clamped between the pathological structure and the canal wall. In most other pathological situations in which nervous tissue is deformed, there is no firm opposing surface, the deformation instead being produced by the pathological structure impinging on the cord that is stretched due to flexion of the spine. The interstitial pressure is then raised and the blood supply is put at risk."

e) "Tension is thus the pathologically significant force, whether generated by a pincer action, by local induration and deformation, or by a space-occupying lesion. Whether or not the tension leads to neurological deficit will depend on its magnitude and its duration of action."

f) "The magnitude of the tension in the cord depends firstly on the anatomical factor of body posture, which determines the relative lengths of the spinal canal and

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