Deformities are variations in the form of the body as a whole, or in one or more of its parts, constituting a departure from the normal conditions of structure, and usually implying a corresponding divergence from natural and healthy functions. They may be divided into three groups, with reference to their origin—the hereditary, the congenital, and the acquired. The first group is characterised by a marked tendency to recurrence in the line of direct descent from generation to generation, as in those cases where the presence of extra fingers or toes has become characteristic of many members of one family (for a full record of an interesting case in point, see Carpenter's Human Physiology). Such hereditary tendencies to modification of form along certain lines of descent constitute local examples of the 'Natural Law of Variation,' which plays an important part in the Darwinian theory of the origin of species. According to this theory, modifications which are of value in the exigencies of the struggle for existence will become perpetuated, and supersede earlier and less favourable conditions of conformation; while modifications that do not possess such value, after repetition through one or two generations, disappear again, and these, since they are useless as well as aberrant conditions, are properly included within the group of hereditary deformities. In the second group, that of congenital deformities, occur those much more frequent abnormalities that result from disturbing influences acting on the otherwise normal embryo previous to its birth. These anomalies occur chiefly as deformities by defective development, and deformities by perverted development. Arrested development may be general, affecting the whole body, when a dwarf is produced; or local, affecting individual organs or parts, producing a great variety of obvious local deformities, in disproportion or malformation of special regions of the body. If the local arrest of development is absolute, the defect caused by it is so great as to constitute a Monstrosity (q.v.), where whole organs may be absent, as the brain (anencephalia), the skull (aerania), the lower jaw (agnathia), all the limbs (amelus), or one or more of them (mono-brachius, monopus). Perverted development is seen in cases where parts normally separate become fused together, as in the 'siren-monster,' in which both lower limbs are welded into one tapering extremity. The causes producing these congenital monstrosities and the lesser and more common deformities of the same class are very varied and often very obscure. Modern writers have, however, made them a subject of special study under the name Teratology (teras, 'monster, logos, 'science'), and have devised experimental conditions which illustrate the subject. Thus, in the case of the embryo chick, Panum found that deformities could be produced by varying the temperature of the hatching apparatus, and varnishing the egg shells; while Dareste discovered that a like result occurs if the eggs be placed vertically instead of lying on their sides; and Gerlach, by varnishing the whole surface of the egg with the exception of a Y-shaped streak on one side, succeeded in producing a double-bodied chicken.
In the human subject the cause of the deformity may depend on purely mechanical conditions, as when the navel-string of the embryo becomes twisted round a limb and causes its gradual separation and destruction (so-called 'intra-uterine amputation'). In many cases, however, the cause is much more recondite, and appears to depend upon a variety of circumstances affecting the maternal organisation. In an increasing proportion of cases which are carefully investigated, it appears that maternal impressions, the result of shock or unpleasant experiences, may have a considerable influence in producing deformities in the offspring. This has for long been a popular theory, and it is one that recent scientific observation is tending to confirm, but only in a comparatively limited proportion of cases; and it must be admitted that the majority of cases cannot be explained on any theory of causation as yet suggested.
The chief varieties of malformation, coming under the heading of congenital deformities, are the following: (1) As regards the number of parts. In the Siren, two lower extremities are fused into one mass, but dissection shows that all the constituent bones of the limbs may be present, though much distorted, in the combined structure. In the Cyclops, the eyes are similarly fused into one irregular structure occupying the centre of the face. (2) As regards the size of parts. This may involve the whole body, as in dwarfs, of whom there have been some remarkable peripatetic specimens: the Corsican fairy was only 2 feet 7½ inches high; Mademoiselle Crachani, the smallest lady who ever lived, died at ten years of age, only 20 inches in height. This kind of deformity is not necessarily hereditary; the father of Borowlaski, who was only 39 inches when thirty years old, had six children alternately short and tall; and dwarf women have brought forth infants as long, when extended, as their mothers. One limb only may be diminutive. Of course, deformities the opposite of these exist, such as giants, or instances of premature or excessive local growth. O'Byrne, the Irish giant, measured 8 feet 4 inches when he died at the age of twenty-two. Such individuals are generally subject to premature decay. One finger or one toe often grows to such abnormal dimensions as to necessitate its removal (local hypertrophy). (3) As regards the shape and continuity of parts. Distortion may occur from partial paralysis or irregular muscular action at an early stage of development, giving rise to Club-foot (q.v.), club-hand, &c.; or natural fissures or apertures which should close in the course of development may remain open, as in harelip, cleft palate, and spina bifida.
Acquired deformities arise in various ways as the result of injury or disease at any period after birth. Among injuries, burns, scalds, fractures, and dislocations are the most fertile causes of permanent deformity; and among diseases, rickets, and other diseases of bone, leprosy, and rheumatic affections are common causes. Another group of these affections, known as 'trade' deformities, are directly traceable to the special work done by the person suffering from them.