We include under this heading lesions of the skin in which, by reason of disease of the nervous system, such changes occur in the nutritive processes as to lead to definite anatomical results. The existence of separate trophic nerves for the skin has not been demonstrated. But there are evidences, especially in pathological processes, that trophic centres exist, and that fibres convey impulses of this kind to the skin. The ganglia of the posterior roots of the spinal nerves and the similar Gasserian ganglion seem to contain trophic centres, and the trophic fibres are either identical with the sensory ones or run along with them.

Causation

We have already seen that certain conditions of the nerves lead to disorders of the circulation in the skin (vide, ante), and that some of these affections graduate towards inflammations. We have here to consider conditions, more or less definitely related to the nerves, in which pronounced structural changes occur, many of them having the characters of inflammation.

The exact relation of the lesions to the nerves is often difficult to determine. On the one hand, it is undoubted that interference with the nerves by division or by irritation induces lesions of the skin Again, locomotor ataxia is often accompanied by cutaneous eruptions especially during the early stage. These are probably the result of the irritation of sensory nerves. Again, there are skin lesions which apparently arise by irritants in the blood attacking the nerves. This possibly applies to some cases of urticaria and erythema nodosum, and is asserted also to have to do with the causation of herpes. On tin-other hand, it is difficult to assign the exact modus operandi of the nerve-condition on the lesions in the skin. The influence may be exercised by means of the blood-supply, but also more directly. It can scarcely be thought that the nerve-lesions in themselves are capable of producing inflammations of the skin, for which the action of irritants is necessary, but nerve-conditions may so affect the skin as that ordinary stimuli, as of friction, etc., may come to act as irritants and induce inflammation. In such cases the nerve-lesion is the most important element in the causation.