OSTEOLOGY AND ARTHROLOGY 77 



minates inferiorly in two convex surfaces, which are separ- 

 ated 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 trochleae ; accordingly, at the level of 

 these articulations, the movements of flexion and extension 

 only can take place. 



In the felidae, 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. 



