Prognosis.—­The rate of growth of the subcutaneous and mixed forms of naevi varies greatly.  They sometimes increase rapidly, especially during the first few months of life; after this they usually grow at the same rate as the child, or more slowly.  There is a decided tendency to disappearance of these varieties, fully 50 per cent. undergoing natural cure by a process of obliteration, similar to the obliteration of vessels in cicatricial tissue.  This usually begins about the period of the first dentition, sometimes at the second dentition, and sometimes at puberty.  On the other hand, an increased activity of growth may be shown at these periods.  The onset of natural cure is recognised by the tumour becoming firmer and less compressible, and, in the mixed variety, by the colour becoming less bright.  Injury, infection, or ulceration of the overlying skin may initiate the curative process.

Towards adult life the spaces in a subcutaneous naevus may become greatly enlarged, leading to the formation of a cavernous angioma.

Treatment.—­In view of the frequency with which subcutaneous and mixed naevi disappear spontaneously, interference is only called for when the growth of the tumour is out of proportion to that of the child, or when, from its situation—­for example in the vicinity of the eye—­any marked increase in its size would render it less amenable to treatment.

The methods of treatment most generally applicable are the use of radium and carbon dioxide snow, igni-puncture, electrolysis, and excision.

For naevi situated on exposed parts, where it is desirable to avoid a scar, the use of radium is to be preferred.  The tube of radium is applied at intervals to different parts of the naevus, the duration and frequency of the applications varying with the strength of the emanations and the reaction produced.  The object aimed at is to induce obliteration of the naevoid tissue by cicatricial contraction without destroying the overlying skin. Carbon-dioxide snow may be employed in the same manner, but the results are inferior to those obtained by radium.

Igni-puncture consists in making a number of punctures at different parts of the naevus with a fine-pointed thermo-cautery, with the object of starting at each point a process of cicatrisation which extends throughout the naevoid tissue and so obliterates the vessels.

Electrolysis acts by decomposing the blood and tissues into their constituent elements—­oxygen and acids appearing at the positive, hydrogen and bases at the negative electrode.  These substances and gases being given off in a nascent condition, at once enter into new combinations with anything in the vicinity with which they have a chemical affinity.  In the naevus the practical result of this reaction is that at the positive pole nitric acid, and at the negative pole caustic potash, both in a state of minute subdivision, make their appearance.  The effect on the tissues around the positive pole, therefore, is equivalent to that of an acid cauterisation, and on those round the negative pole, to an alkaline cauterisation.