I VENTURE to treat of all these under one head, introducing the affections of the ganglia at the end of each chapter, somewhat in the form of an Appendix. To subdivide diseases of the nerves into such as affect the neurilemma, and such as involve the fibres, in the same manner as the affections of the central organs were divided into those of the organs themselves, and those of its membranous investments, is for the most part impossible: for, on the one hand, those elements are in the nerves too intimately united, and on the other, our information on the subject is too defective to permit such a distinction. And not only is it impossible, but if the necessary attention were paid to the subject, the distinction would prove of no real value.

1. Deficiency and Excess of Development - The nerves seem never to be altogether wanting, even in those monsters which are most imperfect; but a single one, or even several, are not unfrequently so. Not only are no nerves developed when the parts which they should supply do not exist, but they are defective, though not perhaps quite absent, when portions of the body are incompletely formed; as when there are limbs, but no muscles within them. The most frequent instances of deficiency are those of the olfactory and optic nerves, when the nose, the eyes, or the whole face are wanting, or arrested in their development. Usually, when no eyes have been formed, or when their development has been arrested, the third, fourth, and sixth pair, and the first branch of the fifth are deficient too. In a cyclops in which the face was almost entirely wanting, no facial nerve could be found; nor can even the lingual branch of the fifth pair, or the hypoglossal, when no tongue is developed. (Tiedemann.) "In monsters born without extremities, or with only imperfect ones, the deficiency of the limbs corresponds exactly with that of the nerves; for when single fingers or toes, a hand or foot, a forearm or leg are wanting, their nerves are wanting too; if there be a deficiency to the extent of a whole limb, the brachial or the lumbo-sacral plexus is small, appears entangled, and gives no branches except to the neighboring part of the trunk. If, with the arm, the shoulder should be absent, or half the pelvis with the lower extremity, their nervous plexuses are absent also, and only some posterior branches for the muscles of the neck, or for the lower part of the dorsal muscles, pass out through the intervertebral foramina. In those monsters, not unfrequently found in the human species, which are without the radius and thumb, the radial nerve ends at the elbow, or merely sends down a few slender threads to the back of the hand." (Otto).

Lastly, there are frequently no nerves in supernumerary parts, especially such as are imperfectly formed.

The absence of a nerve is often only apparent; when the periphery is examined, as it always should be to determine the deficiency absolutely, the nerve will sometimes be found running in one sheath with some other nerve. A nerve sometimes appears to be wanting, when it is only partially so, the trunk of the nerve not reaching the central organ.

Excess of development is illustrated in a plurality of the nerves. An unusual division of a nervous trunk into its branches, or a division of it in an unusually high position, presents merely the appearance of plurality. A real plurality occurs only when there is an increase in the number of other corresponding parts of the body; for instance, there is one spinal nerve more when there is an additional vertebra, and more digital nerves when there is a perfectly organized supernumerary finger In double monsters, the nerves are doubled and divided according to the manner and degree of the duplication; or they are double, but communicate in parts of their course; or they are originally double, then unite, and remain single.

As it has been noticed that the absence of nerves is often only apparent, so may it also be observed that an excess of development is sometimes only apparent too. Sometimes the unusual branching of a nervous trunk gives it the appearance of being developed to an unnatural degree.

Deficiency and excess in the development of the ganglia are exemplified, on the one hand, in their absence or unusual smallness; and, on the other hand, in their presence on nerves where they are not generally found, or in their existence in unusual number or size.

2. Malformations And Anomalies In The Origin, Course, And Branching Of Nerves

Anomalies in the form of the trunk of a nerve are very seldom of any importance; instances of it are met with in the olfactory nerves, which are sometimes hollow at birth; and in the optic, which are found in cyclops monsters to be single, or in various extent fused together.

Malformations of the brain occasion many variations in the origin and course of the nerves, even in individuals who are otherwise naturally formed. The best examples of this are afforded, in cyclopia, by the optic and four following nerves of the eye; by the third nerve when it gives off the long root of the ophthalmic ganglion, and by the sixth, which may supply a nasal branch that is lacking from the fifth. (Otto).

Next to these may be arranged anomalies in the branching of nerves, and in the formation of anastomoses and plexuses, unusually high divisions, splitting of a nervous trunk in the form of islands, deficient, multiplied, uncommon anastomoses, etc.