THE FIRST PHALANX 101 
consisting of two glenoid cavities separated by a sagittal groove; the medial cavity 
is a little larger than the lateral one. On each side is a buttress-like tuberosity for 
ligamentous attachment. The dorsal surface has a slight elevation for the attach- 
ment of the extensor tendons. 
The distal extremity (Extremitas distalis) is smaller, especially in its dorso- 
volar diameter. It presents a trochlea for articulation with the second phalanx, 
consisting of a shallow sagittal groove separating two condyles; the medial con- 
Intermediate groove — 
Proximal articular surface ~~ 
Tuberosity 
Eminence for extensor tendons —— 
First phalanx 
Dorsal surface 
Eminence for collateral ligament 
Distal articular surface ° 
Proximal articular surface - 
Dorsal surface - Second phalanx 
Eminence for collateral ligament 
Distal articular surface 
Articular surface . 
Angle --- 
a 
: ¥ 
Depression for collateral (4% 
ligament ~ 4 
a 
Dorsal groove —* 
Coronary border -- 
Articular surface --- 
Third phalanx 
(a 
Extensor process -Y bi 
Dorsal surface —-= 
Distal border 2 Ne ‘| 
Fic. 79.—PHALANGES AND Disrat SesaMorp oF Horst; Dorsat ASPECT. 
dyle is a little the larger, and the two are separated posteriorly by a notch. On 
either side, just above the margin of the articular surface, is a depression sur- 
mounted by a tubercle, to both of which the collateral ligament is attached. Be- 
hind the tubercle is a distinct mark to which the superficial flexor “tendon is 
attached. 
Development.—The first phalanx ossifies from three centers. The distal end 
unites with the shaft before birth, the proximal end at about one year of age. 
The first phalanx contains a small medullary cavity in the middle of the shaft. It may be 
