=The Metacarpo-phalangeal Articulations.=--With regard to the mobility, it is in these articulations, as in those of the wrist--that is to say, although in all quadrupeds the first phalanges can be flexed and extended on the metacarpus, it is only in the cat and dog that lateral movement is possible. Indeed, in the horse, in which the principal metacarpal terminates inferiorly in two convex surfaces, which are separated by a crest; and where the whole articulates with a cavity on the superior extremity of the first phalanx; because of the hinging of these surfaces, there can only be movements of opening and closing of this articulation. The first phalanx is directed backwards during flexion and forwards during extension. In the dog and the cat the digits can be separated from each other, and also drawn together--that is to say, abducted and adducted; but, as in man, these movements can be made only when the first phalanges are in the state of extension. During flexion they are impossible, because of the tension of the lateral ligaments, which increases as the flexion is more pronounced. This can be demonstrated, for example, in the cat, which, in order to separate the digits, opens the hand widely--that is to say, forcibly raises the first phalanges. =The Interphalangeal Articulations.=--The phalanges are in contact with one another by surfaces, which, on one side, are of trochlear form, and, on the other, are moulded on these trochleæ; accordingly, at the level of these articulations, the movements of flexion and extension only can take place. In the felidæ, the claws which the third phalanges bear cannot be utilized when the latter are in a state of extension, at which time, being forcibly raised, they are, in fact, placed on the outer sides of the phalanges, which are grooved to receive them. But when the animal wishes to use them, it flexes those third phalanges, of which the terminal extremity is then projected forward, and the claws are ready to fulfil their function. But at the same time it extends the first phalanges, to produce a certain tension of the flexors of the digits, and thus enable the latter to act with greater efficacy, with a minimum of contraction. We can demonstrate this action experimentally on ourselves. It is enough to carry the first phalanges forcibly into a state of extension; the third phalanges then become flexed, quite spontaneously, by the tension of the tendons of the flexors which are inserted into them. At the same time, if we examine the felidæ which we have taken as examples, when the first phalanges are in the state of extension, the digits will be found to be separable, as we have already indicated in connection with the metacarpo-phalangeal articulations, with the result that the claws are then able to lacerate a wider surface.