640 MAMMALIAN ANATOMY 



on the dorsal aspect of the bone. The sides of the head are circular 

 and impressed by a pit for the attachment of ligaments. 



PECULIARITIES OF THE PROXIMAL PHALANGES. 



The proximal phalanx of the third digit is the largest ; the others 

 decrease in size in the following order : the fourth, the fifth, and the 

 first or proximal phalanx of the thumb. The third and fourth have 

 the same shape, the second and fifth are bent slightly toward the 

 median line, and the median end of the trochlea is prolonged. 



The proximal phalanx of the thumb is very short, and the articular 

 surface on the base is shallow ; the shaft is twisted about its long axis, 

 the head facing more to the dorsal side than does the base, thus correct- 

 ing the radio-dorsal position which the metacarpal sustains to the rest 

 of the metacarpus and permitting the terminal phalanx to move in a 

 dorso-palmar plane almost parallel with the planes of action of the 

 phalanges of the other digits. As there is no middle phalanx to the 

 thumb, the head articulates with the distal phalanx. The articular 

 surface of the head is transversely cylindrical, and lacks the dorso- 

 palmar groove which forms a trochlea on the proximal phalanges of 

 the other digits. 



MIDDLE PHALANGES. 

 CHARACTERS COMMON TO ALL. 



Every middle phalanx is narrower and almost a fourth shorter 

 than the corresponding proximal phalanx. It presents a proximal 

 extremity or base, a shaft, and a distal extremity or head. 



The Base is prismatic ; narrow dorsally and expanded laterally and 

 toward the palmar aspect. It has four surfaces, the proximal, the 

 palmar, the radial, and the ulnar. 



The proximal surface of the base has the outline of an equilateral 

 triangle, one angle dorsal and the opposite side palmar. The palmar 

 margin is emarginate, and the lateral margins are arcuate. Of the 

 angles the dorsal is distinct and prolonged and the lateral are flat and 

 slightly rounded. A well-marked median ridge runs from the dorsal 

 angle to the palmar side. It is convex transversely and concave in the 

 dorso-palmar direction to fit into the groove in the trochlea of the 

 head of the proximal phalanx. The surface on each side of this ridge 

 is concave in both directions, and is applied to the convex lateral part 

 of the trochlea of the proximal phalanx. 



