(5) Redundancy and Defect in Single Parts
The simplest case of this redundancy is a sixth digit, well formed, and provided with muscles (or tendons), nerves, and blood-vessels like the others; it is usually a repetition of the little finger or toe, and it may be represent one or both hands, or one both feet, or in all four extremities, as in the giant of Gath. The want of one, two, or more digits on hand or foot, or on both, is another simple anomaly; and, like the redundancy, it is apt to repeat itself in the same family. Meckel saw a girl who had extra digit on each extremity, while a sister wanted four of the fingers of one hand. Where the supernumerary digits are more than one on each extremity, the whole set are apt to be rudimentary or stunted; they look as if two or more of the embryonic buds had been subject to cleavage down the middle, and to arrest of longitudinal growth.
There are two or three authentic instances of a whole lower limb appearing at birth as two withered halves, as if from embryonic cleavage. (Footnote 763-1)
Other redundancies of the skeleton are extra vertebrae (sometimes the coccygeal, giving the appearance of a rudimentary tail), or an extra rib.
A double row of teeth is occasionally met with; the most interesting case of this anomaly is that in which the rudiments of a double row exist from the first, but the phenomenon is sometimes produced by the milk teeth persisting along with the second set. One or more extra teeth are occasionally met with in line with the rest.
Among redundancies of the parts, by far the most frequent is an extra nipple, or pair of nipples. It is only the nipple, or the most external mechanical adjunct of the mammary apparatus, that is repeated, and very seldom, if ever, the breast structure itself. The nipple, although it is the latest addition to the mechanism of lactation, is in the individual mammal developed on the skin before the gland is formed underneath; and that facility, which applies to the development of external characters generally, appears to be the reason why there may be one or more extra nipples but no redundant gland. In the same connexion, it is interesting to observe that the supernumerary nipple has been shown by statistics on a large scale to be twice as common in men as in women, although in the male the mammary function never comes to maturity, and even the structure retrogrades after puberty. Traces of an additional nipple or pair of them, in more or less symmetrical position below the normal ones, are not very uncommon when carefully looked for.
Among the sense organs there is a remarkable instance recorded of doubling of the appendages of the left eye, but not of the eyeball itself ; the left half of the frontal bone is double, making two eye-sockets on that side, and the extra orbit has an eyebrow and eyelid. (Footnote 763-2)
The external ear (pinna) has also been found double on one side.
Doubling of any of the internal organs is extremely rare, and is probably always traceable to a more or less complete fissuring or lobation. The ducts or vessels connected with organs, and playing a purely mechanical part, are not unfrequently doubled; thus each kidney may have two ureters, and a similar variation may occur in veins and arteries.
(763-1) See Förster's Atlas, Taf. viii., figs. 13 and 14.
(763-2) See preparation in the Würzburg Museum, figured by Förster, Taf. viii., figs. 9-12.
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Abnormal Development & Congenital Malformations - Table of Contents