appendage, the nature of which may only be indicated by
the presence of a rudimentary nail; sometimes it contains bone
representing one or more phalanges, or it may be fully formed (Fig.
175). In the majority of cases the superfluous finger should be
removed.

[Illustration: FIG. 175.--Supernumerary Thumb.

(Photograph lent by Sir George T. Beatson.)]

#Congenital Deficiencies in the Number of Fingers.#--One or more
fingers may be absent, such deficiency being often associated with
imperfect development of the radius or ulna; or they may be
represented by short rounded stumps, which are ascribed to the
strangulation of the digits by amniotic bands _in utero_--the
so-called intra-uterine amputation.

#Webbing of Fingers (Syndactylism).#--Congenital webbing or fusion of
the fingers may be associated with polydactylism or with congenital
hypertrophy, and, like other digital deformities, may affect several
members of the same family. The degree of fusion ranges from a web of
skin joining the fingers to a fusion of the bones, the latter being
well seen in skiagrams. If an operation is decided upon, it should not
be performed until the age of five or six years. In the simplest cases
it is only necessary to divide the web and to unite the cut edges of
skin along each finger by sutures, a skin graft being inserted into
the angle between the fingers. An operation in which the skin is
dissected up in the form of flaps may be required, but it should not
be lightly entered upon, as in young children it has been known to be
followed by gangrene of one or more of the digits.

#Congenital Hypertrophy of the Fingers.#--This is a form of local
giantism affecting one or more digits, and involving all the tissues.
The finger is usually of abnormal size at birth, and continues to
grow more rapidly than the others, and it may also come to deviate
from its normal axis. Such a finger should be trimmed down or removed,
to permit of the use of the other digits.

#Trigger Finger# (Fig. 176).--This is an acquired condition in which
movement of a finger or thumb, either in flexion or extension, is
arrested, and is only completed with the assistance of the other hand.
The obstacle to movement is usually overcome with a jerk or snap
suggesting a resemblance to the trigger of a gun or the blade of a
clasp-knife. The commonest cause is a disproportion between the size
of the tendon and its sheath, such as may result from a localised
thickenin