A neuroma is properly a tumour composed of nerve tissue. As there are two kinds of nerve tissue so we may distinguish a ganglionic and a fibrous neuroma. There are some cases of small tumours projecting from the cortex of the brain which may be named ganglionic neuromas, but they are excessively rare, and the name neuroma is virtually reserved for tumours of nerve stems.

All tumours of nerve stems are usually designated neuromas, but as such tumours may really be composed of fatty, mucous, or fibrous tissue, it has become customary to distinguish those which actually contain new-formed nerve fibres as true neuromas, and the others as false neuromas.

The true neuroma is composed of nerve fibres, which may be either medullated, as in the ordinary cerebro-spinal nerve, or non-medullated.

One of the most striking forms is the so-called Amputation neuroma (Fig. 85), which is a very frequent lesion in stumps. Dr. Sutherland has found them 11 times out of 11 cases. The tumours are usually multiple, and form bunches of little knobs, which have a hard consistence, and to the naked eye look fibrous. Under the microscope medullated nerve fibres are found running in bundles, but there are also many fine fibres which are probably non-medullated nerve fibres. It is in this case as if the cut end of the nerve had made an attempt at regeneration of the lost portion. Allied to this form is the traumatic neuroma, occurring as the result of an injury in the course of a nerve.

But neuromas occur in the course of nerves spontaneously, and they are often multiple, forming oval swellings, hard and fibrous in appearance. They contain much fibrous tissue, in the midst of which there are medullated nerve fibres (see Fig. 86) recognizable in the fresh state by the double contour, especially when the connective tissue has been rendered transparent by acetic acid or liquor potassse. But usually in addition to these medullated fibres, there are fine nucleated es, which Virchow regards as non-medullated nerve fibres. In some neuromas these are very abundant. An interesting but rare form is the Plexiform neuroma. In it the nerves in an area of the skin become enlarged, so that the part to the touch feels like a congeries of worms. The enlarged nerves vary from the size of a crow-quill to that of the thumb. The tumours are mostly congenital.

Fig. 80. - Amputation neuromata in a stump. The internal and external popliteal nerves are involved. The amputation was ten years before.

Fig. 86. - Transverse section of a neuroma. The medullated nerve fibres are shown, the appearance being much like that of the section of a nerve, x 80.

There is no doubt that most tumours of nerves are really false neuromas. This applies, as we have seen at p. 212, even to the multiple tumours which have been usually regarded as the typical neuromas. It applies also, at least in part, to the amputation neuromas. It is obvious that as a nerve stem is composed of connective tissue and nerve fibres, it must be often very difficult to determine whether there has been new-formation of nerve fibres or not. It seems questionable whether it is worth while, for the sake of consistency in nomenclature, to attempt to restrict the term neuroma to those actually composed of nerve tissue.

Literature

Vibchow, Geschwulste, iii., 233; R. W. Smith, On Neuroma, 1849; see also New Syd. Soc. Reprint. Wood, On painful subcutaneous tub., 1812; Paget, Lect. on surg. path., 3rd ed., p. 490. On false neuroma, see especially Recklinghausen, Ueber multiplen Fibrome, 1882; Kriege, Virch. Arch., cviii.; Westphalen, Virch. Arch., ex.; Philippson, Virch. Arch., ex. Plexiform neuroma - Bruns, Virch. Arch., 1., 1870; Lecroix and Bonnauh, Arch, de med. exper., ii., 1890.