642 MAMMALIAN ANATOMY 



the head having been turned to the ulnar side. It bears a smooth 

 articular surface, which is nearly straight transversely, except at the 

 palmar side, where it is slightly concave, and strongly convex from the 

 dorsal to the palmar side ; the ends are flattened, or depressed, and 

 roughened for the attachment of ligaments. The head forms a hinge- 

 joint with the concavity on the base of the distal phalanx. 



PECULIARITIES OF THE MIDDLE PHALANGES. 



Albeit all the middle phalanges are similar, the third and fourth, 

 nevertheless, are of nearly equal length and about a fifth longer than 

 the second and fifth, which are also nearly equal. The relation of the 

 length to the width also varies, the median two being longer and more 

 slender, the lateral two shorter and stouter. There is no middle 

 phalanx to the first digit, or thumb. 



DISTAL PHALANGES. 



The Distal Phalanges (Fig. 508) are very different from the others. 

 They are strongly compressed from side to side, high from the dorsal 

 to the palmar border, and very short from the proximal to the distal 

 end. On the distal surface is a thin curved vertical plate which 

 supports the horny claw. The proximal end is the base ; the central 

 portion is the shaft, although it has lost the elongated form common 

 to the shafts of long bones ; the curved plate is the head. The phalanx 

 has four surfaces. 



The proximal surface is a narrow strip with a wide palmar side 

 and ending in a rounded dorsal point above. It is deeply concave in 

 the dorse-palmar direction, and almost flat from side to side. The 

 palmar two-thirds are occupied by a shallow articular cavity for the 

 head of the middle phalanx ; the dorsal rounded third serves for the 

 attachment of strong ligaments. 



Of the margins of the radial and ulnar surfaces, the proximal is 

 emarginate, the dorsal and distal are arcuate, and the palmar is straight. 

 In the proximo-distal direction the surfaces are wide at the dorsal and 

 narrow at the palmar border. They are gently convex, and extended 

 distally and dorsally to form a hood, between which and the claw-plate 

 is inserted the base of the claw. The palmar border is a small, rough, 

 depressed quadrate or oval area, to which is fastened the tendon of the 

 flexor profundus digitorum muscle. The dorsal border is much longer, 

 and is convex in both directions. 



