sensibility in uncomplicated dissociation. Similar principles guided our choice in examples of cortical and subcortical lesions.
After this laborious analysis of disordered function in patients with some stable lesion, we were able to interpret the less satisfactory observations which were alone possible in those who suffered from some progressive or paroxysmal affection.
Careful selection of the examples subjected to intensive study and this dual attitude towards the clinical material at our disposal is necessary before we can hope to discover the meaning of disordered functions in the nervous system.
4. The Importance of Residual Sensibility.
Sherrington (108) first introduced a valuable means of investigating sensation, known as the method of "Residual Sensibility." When he wanted to determine the extent of skin supplied from the fourth thoracic root he destroyed several roots above and several below, whilst the fourth remained intact. This left a sensitive area in the centre of a zone of anaesthesia; every part that received its innervation, however slightly, from the fourth thoracic was marked out by residual sensibility.
The value of this method and its universal importance has been strangely overlooked by neurologists. They continue to publish reports in which the extent of the analgesia is solemnly discussed; but they do not seem to recognise the importance of considering what parts still retain their sensibility. Suppose the seventh and eighth cervical and first thoracic roots have been destroyed in man; certain portions of the upper extremity become insensitive to prick. But when we have carefully determined the extent of the analgesia we are not justified in assuming that it represents the full supply of the divided nerve roots; the area of sensory loss corresponds solely to those parts of the limb which they innervate exclusively. On the other hand, the upper or headward border of the loss of sensation corresponds to the lower limits of the sixth cervical root, which is intact; similarly the post-axial limits of the analgesia mark out the upper limits of the second thoracic. It is not the analgesia, but the extent of the residual sensibility that is significant in such a case.
The same principle applies to the loss of sensation produced by lesions of the spinal cord. All the diagrams constructed to show sensory segmentation are built up on the study of analgesia; in each case the borders are carefully determined and transferred to a chart as the limits of the highest segment affected. In realty the sensory condition should be looked at from the opposite point of view; the upper border of the analgesia corresponds to the caudal extension of the lowest unaffected portion of the spinal cord. On the other hand, the loss of sensation corresponds to those parts of the body which are exclusively supplied from below the lesion, a matter of little scientific importance.
This perverted outlook is responsible for much faulty diagnosis; for it is more important to know what segments are still capable of exercising their
