The two principal divisions of herpes are to be distinguished in their general causation, although similar in their ana tomical characters. On the one hand, Herpes facialis and Herpes genitalis occur in the course of a number of acute febrile diseases, sinh as pneumonia, cerebro-spinal meningitis, and malaria, being particularly frequent in pneumonia. It is probable that the infective agent acts on the nerves of these parts, and so leads to the affection. On the other hand. Herpes zoster is related to an unknown irritant which attacks the particular nerve stems, producing neuralgia. The eruption is limited to the distribution of particular nerves, and is strictly unilateral. It is frequently related to an intercostal nerve, or it may occur on one side of the face in connection with a neuralgia of the fifth nerve.

The eruption in the skin is in the form of vesicles which somewhat rapidly pass through a series of changes. There is first a group of slight elevations occupying a reddened patch of skin, and each of these rapidly develops a vesicle in a few days the vesicle dries up into a crust, under which fresh epidermis is formed. In its details the eruption is inflammatory. There is hyperemia and oedema of the papillary layer of the skin. The serous exudation collects in the Malpighian layer of the epidermis, separating its cells just as in the case of the small-pox eruption. The vesicle is divided by a network composed of the elongated and contorted epidermic cells, which often present clear spaces or vacuoles in their substance. In the serous contents of the vesicles there are leucocytes, and these may accumulate till the fluid approaches to the nature of pus. The papillary layer is also frequently the seat of infiltration with round cells. Occasionally there is haemorrhage, and the papillary layer of the skin may be destroyed, so that when healing occurs by granulation a cicatrix may be the result.

The nervous origin of herpes has been abundantly established. Direct injury to a nerve,[as by a gunshot or other wound, may cause it. It is to be presumed that, in this case, the inflammatory process in the wound irritates the nerve, and the cutaneous inflammation may be regarded as due to the irritation of trophic fibres.

A herpetic eruption may arise from disease of the spinal cord and brain. In the case of the cord it is chiefly in locomotor ataxia that we meet with herpes, and the eruption is accompanied by the severe pains characteristic of that disease. We know that in locomotor ataxia it is the posterior columns of the cord that are affected, and this would indicate that the cutaneous trophic fibres run in these parts, while the coincidence of neuralgic pains also indicates a sensory locality. It is here again to be presumed that the lesion producing the herpes is an irritative one as indicated by the violent coincident pains. But we may have herpetic eruptions from disease in the brain. There are cases recorded in which one half of the body was affected, and in some of these there was a hemiplegia due to a coarse lesion in the brain. There are also a few cases on record of universal or nearly universal herpes.