The true neuromas are again subdivided into those in which the nerve-tissue composing them resembles exactly the fibres of the peripheral nerves, showing with the microscope the double-contoured white substance of Schwann surrounding an axis-cylinder, and those in which the tumor is made up of fibres which Virchow has shown to be non-medullated nerve-fibres—i.e. the axis-cylinder without the white substance of Schwann. These two forms have been distinguished by the names myelinic and non-myelinic. The true neuromas are non-malignant, although showing the tendency to recur after extirpation, are of slow growth, and as a rule do not increase to a very great size. The best type of the myelinic neuromas is found in the spherical or spindle-shaped enlargements at the cut ends of nerves, particularly in the stumps of amputated limbs, where they are found oftenest intimately connected with the cicatricial tissue, though sometimes lying free. They consist of true medullated fibres mixed with some fibrous tissue. The fibres composing them are derived partly from splitting up and proliferation of the fibres of the nerve itself, partly are of new formation, the appearances strongly recalling the process of regeneration in nerves. Myelinic neuromas consist of fibres and nuclei so closely resembling in microscopic appearance the fibromas that they have hitherto been confounded with them; and there is a difference among the highest authorities as to the certainty of their diagnosis, and, in consequence, of the frequency of their occurrence. The true neuromas may include in their structure all of the fibres of the nerve-trunk or only a portion of them (partial neuroma)—a fact of importance in their symptomatology. Of the false neuromas, the fibromas are by far the most frequently met with. They appear as knots, more or less hard, upon the course of the nerve-trunk, which they may involve completely or partially. They are often excessively painful to the touch or spontaneously, most of the so-called tubercula dolorosa belonging to the fibro-neuromas. Fibromas sometimes occur along the trunk and branches of a nerve, forming a plexus of knotted cords (plexiform neuroma). Fibro-sarcomas are not an infrequent form of neuroma.
Myxomas often occur upon the peripheral nerves, and are frequently multiple, their points of predilection being the larger trunks, as the sciatic, ulnar, etc. They show their characteristic soft structure, and are usually spindle-shape, assuming a rounder form as they attain a large size. The various forms of sarcoma occasionally form tumors upon the nerves, attacking generally the large trunks. Carcinomatous tumors beginning upon the nerves sometimes occur, but as a rule these growths involve the nerve by extension to it from adjacent parts.
Syphilitic gummata have been found almost exclusively upon the intracranial portion of the cranial nerves.
Gliomas appear to affect only the optic and acoustic nerves. Lepra nervorum (lepra anæsthetica) produces usually a spindle-form thickening upon the nerve-trunks, but sometimes there are more distinct knots, which may be felt beneath the skin, bead-like, along the course of the nerves of the extremities.
Like the true neuromas, the false neuromas, developing from the neurilemma and perineurium, may involve the whole or only a part of the fibres of a nerve, or the nerve-fibres may run at the side of the tumor—different conditions, which may alter materially the effects produced upon the nerve.
Neuromas, both false and true, may occur not only singly, but often in large numbers, many hundreds having been counted upon an individual. Sometimes they are numerous upon a single nerve-trunk and its branches, and again they may appear scattered over nearly all of the nerves of the body, even to the cauda equina and roots of the nerves. According to Erb,9 isolated neuromas are more frequent in females, while multiple neuromas are found almost exclusively in men. Neuromas vary greatly in size, as we might expect from the very great difference of their nature and structure; sometimes no larger than a pea, they may attain the size of a child's head.
9 Ziemssen's Handbuch.
ETIOLOGY.—In cases of multiple neuromata it would seem as if there was a constitutional condition or diathesis as the foundation of the affection. This we may the more readily believe as there appears good evidence to show that the tendency to the formation of these nerve-tumors is sometimes hereditary, and some of them are congenital.
Idiots and cretins have been observed to suffer in undue proportion with multiple neuromas. We find a direct exciting cause of neuromas in mechanical injuries of nerves, wounds, blows, pressure, etc. Thus, as has been already seen, true neuromas occur in the divided ends of the nerves after amputations or otherwise where a nerve-trunk has been divided (cicatricial neuroma). As such neuromas are in some degree the result of inflammation, it is probable that they may sometimes be caused by chronic neuritis.