These anomalies relate to the thickness of the nervous cords. A palpable increase of all the nerves beyond their natural bulk is extremely rare; a few instances of it only having been observed in idiots, either affecting all the nerves uniformly, or predominating in the sympathetic system. It is more frequently met with in a single nervous trunk.

It has various causes, but the most common are inflammation, inflammatory products, and morbid growths. A more important question is that relating to the increase in size which nerves acquire by hypertrophy.

There is no proof that hypertrophy of a nerve consists in the formation of new fibres in addition to those of which it was originally composed, and the present state of our knowledge opposes any such explanation. It might be, indeed, that the nerve-tubes become enlarged by an increase of their contents; but this, also, is a mere hypothesis, which, although in some degree supported by the enlargement of the tubes, which Herrmann Nasse has observed at the proximal end of a divided nerve before it is completely restored, can yet receive no practical proof of its correctness, even from a future generation. Most probably the neurilemma of the nerve is the part which is hypertrophied. Such we must, for the present, consider to be the state of the enlarged nerves of hypertrophied organs, those, for instance, of a hypertrophied heart.

There might, however, be a true hypertrophy of the ganglia, an aggregrating of new ganglion cells; although of it also we lack proof from facts. I have met with a case of general emaciation, combined with an eminent degree of hypochondriasis, in which the central abdominal ganglia were considerably enlarged.

It is a peculiarity frequent in individual conformation, that the nerves are more or less manifestly thin and slender. Very commonly when the brain is hypertrophied, there is a striking thinness of the cerebral nerves. The nerves, too, of parts which are developed but imperfectly, or not at all, are thin from the first: an instance of this is met with in the optic nerves in cases of microphthalmus.

The nature of atrophy is more distinct than that of hypertrophy. Sometimes it is confined to the nerve-tubes, sometimes it involves the neurilemma also. There are various conditions under which it occurs.

1. It is only when general emaciation has advanced to an extreme degree that genuine atrophy of the nerves can be shown to exist with it. In ordinary cases the diminution in their size may be referable to the loss of their natural moisture, and, after that, to wasting of neurilemma.

2. Pressure and stretching often produce atrophy of a nerve, even to such an extent as gradually to destroy it in its whole thickness. We see force of this kind exerted upon nerves, morbid growths, aneurisms, and enlarged organs: especially we might instance the bronchial plexuses of the vagus nerve, which sometimes become so wasted from the presence of tuberculous bronchial glands, that their continuity is at last destroyed. The nerve is at first not unfrequently flattened, its fasciculi, spread abroad, and at length it disappears altogether at the compressed spot. Above, and below, it retains its natural thickness, though, probably, only for a certain time. This form of atrophy evidently affects the whole nerve equally, the nerve-tubes being at first diminished and compressed, and in the end absorbed.

It can be understood that inflammatory products in the neurilemma of a nerve may press upon the tubes and interfere with their nutrition, and so lead to their becoming atrophied: thus, while a nerve is actually increasing in size, its essential element may be diminishing.

3. A nerve may be atrophied in consequence of the wasting and loss of the peripheral or central organs to which it belongs, in fact, in consequence of a permanent solution of its continuity. Frequent examples of such atrophy may be adduced. On the one hand, the optic nerve shrinks because the globe of the eye is wasted, and the nerves diminish in size after the removal of a peripheral part by amputation; while, on the other hand, they are found to lessen in volume when their central organs have wasted; and, again, they are in an atrophied condition in hemicephalus, hydrocephalus, etc, at least within the skull. In these instances it is unquestionably the nerve-tubes which are affected; and all of them, or only a few are affected, according to the amount of wasting, or loss of substance at the periphery or the centre, which is the occasion of the atrophy. Under certain circumstances, nerves, which are extremely atrophied, acquire a translucid grayish-red appearance, especially within the skull. The coloring is produced by the presence of a blastema, filled with numerous nuclei, which, at first gelatinous, and afterwards tough and elastic, takes the place of the nerve-tubes as they disappear, and becomes more distinctly visible as the original neurilemma of the affected nerve diminishes. The Vessels of a nerve in this condition are often palpably dilated.

4. We not unfrequently observe a withered state of the ganglia, a shrinking and leather-like toughness of them, while their color has either altogether disappeared, or is changed to a rusty brownish, yeast-yellow, fawn, or slate gray. It appears frequently to be a primary affection; but in many cases it is a secondary consequence of previous disease. The chief example of it is the wasting of the abdominal ganglia, which follows typhus, and forms one of the few causes which can be found by the knife of the anatomist, for the sickly state succeeding typhus (Typhus-siechthum).