638 MAMMALIAN ANATOMY 



PEOXIMAL PHALANGES. 

 CHARACTERS COMMON TO ALL. 



The length of every proximal phalanx is a little more than four 

 times greater than the average width ; the thickness is less than the 

 width. Compared with the metacarpal it appears stout, as the width 

 is about the same as that of the metacarpal, while the length is only 

 one-half. 



It has a proximal end or base, a shaft, and a distal end or head. 



The Base (Fig. 507) has transverse and antero-posterior diameters 

 slightly greater than those of the shaft. 



The proximal surface has a deeply emarginate palmar border, 

 and is entirely occupied by a concave surface which articulates with 

 the convex head of the metacarpal. The shape of this concavity is 

 crescentic, the arcuate border being dorsal. It faces proximally and 

 dorsally. 



The palmar surface is slightly concave in both directions, and is 

 continuous with the surface of the shaft. Its proximal border is in- 

 cised. The lateral borders are prominent at the proximal end where 

 they afford attachment for the sesamoid bones ; distally they slope 

 toward the long axis of the shaft. 



The sides of the base are gently convex, and marked by the inser- 

 tion of interosseous muscles and ligaments. They slope dorsally and 

 distally into the dorsal surface of the shaft. 



The dorsal surface is convex transversely and continuous with the 

 surface of the shaft. 



The Shaft is slightly arched from the proximal to the distal end. 

 The lateral borders are almost straight, and lie near the palmar aspect 

 of the bone ; they separate the dorsal and palmar surfaces. For most 

 of their extent they are rounded, but at the distal end they present an 

 inconspicuous rough area for ligamentous attachment. 



The dorsal surface is convex in both directions ; the palmar surface 

 is flat transversely and concave from base to head. 



The Head of the proximal phalanx is a cylinder placed trans- 

 versely and deeply grooved in the middle line to form a pulley-like 

 articular prominence or trochlea, which fits into the articular concavity 

 in the base of the middle phalanx. The trochlea is entirely visible 

 on tne palmar aspect where it rises above the plane of the palmar sur- 

 face of the shaft. A small portion only of the articular surface is seen 



