The physiological variations in the range of movements are thus to be explained by variations in the relative length of the anterior fibres of the lateral ligaments. The ideal constitution of a joint depends upon the existence of a certain ratio between the growth of bone and that of ligament. Should the ligaments grow in excess, their redundant length will permit great super-extension, and may even cease to check the movement; but if the bone grow relatively faster than the ligaments, the anterior portion of the latter will the sooner become tense during extension, and where this disproportion is exceptionally great the motion may be checked before it attains physiological completeness, the result being a “hammer finger.” Irregularities of development are most likely to occur in those joints which, for one or other reason, have the least functional activity. In the hand the little finger is much less powerful than its fellows; and in association with this it may often be noticed that the fourth tendon of the flexor sublimis is reduced to a mere thread; in the foot the same thing is observed in the corresponding digit, but in a more marked degree, and it is the degenerate little toe which is most liable to the “hammer deformity.” We may then define hammer finger as the result of a developmental irregularity of the first or second inter-phalangeal joint (rarely of the metacarpo-phalangeal joint) by which the anterior fibres of the lateral ligaments become prematurely tense during extension, and so check that movement before it attains its normal physiological limit. It is precisely analogous to hammer toe; but it is of less frequency than the latter affection, because while civilisation sedulously cultivates the freedom and precision of action in the fingers, it devises foot-coverings to repress the natural play of the toes. The tendency to the deformity may be transmitted by descent through an indefinite number of generations. _Diagnosis._—Spurious hammer finger, like false hammer toe, may occur from—(1) articular lesions due to rheumatism, rheumatoid arthritis, gout, tuberculosis, and inflammations of traumatic origin; or (2) from interference with the muscular functions by paralysis of the extensors or by spastic contraction of the flexors. In the first group the joint will be found in a more or less complete state of ankylosis, movements in all directions being impeded. In the second group the articulation, although contracted, is freely mobile under passive force, unless, as in some congenital paralyses, irregularities of development in the articulations be superadded.