VASO‐DILATOR AXON‐REFLEXES

1. The dilatation of the vessels seen in the initial stages of the inflammatory process does not occur if the region to which the irritant is applied be rendered completely insensitive by means of a local anaesthetic. 2. This vaso-dilatation must, therefore, be the result of nervous impulses which pass centralwards up a sensory fibre or fibres and peripheralwards down a fibre or fibres subserving vaso-dilatation. 3. Section of the spinal cord does not influence this reflex, therefore the centre cannot be situated above the cord. 4. Section of the posterior sensory roots central to their ganglia of origin does not influence the reflex, therefore the centre cannot lie in the spinal cord. 5. The vaso-dilatation is also not interrupted by section of sensory efferent fibres peripheral to their ganglia of origin provided that the severed peripheral portion of the fibre be not degenerated. 6. If the severed peripheral portion of the fibre be allowed to degenerate, the application of an irritant is not followed by any of the usual vascular phenomena characteristic of inflammation. 7. The afferent sensory posterior root fibres must be assumed to undergo a peripheral bifurcation, one limb of the bifurcation passing to end in a sensory end-organ in the skin; this normally conducts sensory impressions towards the nerve-centres; the other limb passes to the vessels of the part irritated, and normally conducts vaso-dilator impulses to them. 8. After section of the nerve-fibres between the ganglia and the periphery, the nervous impulses producing dilatation of vessels in the initial stage of the inflammatory process are limited to the two limbs of this bifurcation. The impulses pass up the one limb and down the other. 9. The effect thus produced is of the nature of an axon-reflex, comparable to those already described in connexion with the sympathetic system. 10. The identity of the vaso-dilator fibres with the afferent posterior root fibres receives strong support from the above experiments.