THE PHALANGES THE FIRST PHALANX THE SECOND PHALANX 89 



at birth. Fusion of the middle part of the shaft with the large metacarpal bone is 

 common. 



THE PHALANGES 

 The First Phalanx 



The first phalanx (Phalanx prima)^ is a long bone, situated between the large 

 metacarpal bone above and the second phalanx below. It is directed obliquely 

 downward and forward, forming an angle of 50 to 55 degrees with the horizontal 

 plane in well-formed limbs. It consists of a shaft and two extremities. 



The shaft (Corpus) is wider and much thicker alcove than below, and presents 

 two surfaces and two borders. The anterior or dorsal surface is convex from side 

 to side and smooth. The posterior or volar surface is flattened, and bears a triangu- 

 lar rough area, bounded laterally l)y ridgc^s which begin at the tuberosities above 

 and converge below; this area furnishes attachment to the inferior sesamoiclean 

 ligaments. The borders, internal and external, are rounded and have a rough area 

 or a tubercle on their middle parts. 



The proximal extremity (Basis) is relatively large. It bears an articular sur- 

 face adapted to the distal end of the large metacarpal bone, consisting of two glen- 

 oid cavities separated by a sagittal groove; the inner cavity is a little larger than 

 the outer one. The posterior angles are formed by buttress-like tuberosities for 

 ligamentous attachment. The anterior surface has a slight elevation for the at- 

 tachment of the lateral extensor tendon. 



The distal extremity is smaller, especially in its antero-posterior diameter. 

 It presents a trochlea for articulation with the second phalanx, consisting of a 

 shallow central groove and two lateral convex areas or condyles; the inner area is a 

 little the larger. On either side, just above the margin of the articular surface, is a 

 depression surmounted by a tubercle, to l)oth of which the lateral ligament is at- 

 tached. Behind the tubercle is a distinct facet to which the superficial flexor ten- 

 don is attached. 



Development. — The first phalanx ossifies from three centers. The distal end 

 unites with the shaft before birth, the proximal end early in the first year. 



The first phalanx contains a small medullary canal in the middle of the shaft. It may be 

 remarked that the bone is twisted slightly; when placed volar surface down on the table, it 

 touches the latter by three points only, the proximal tuberosities and the internal condyle. 



The Second Phalanx 



The second phalanx (Phalanx secunda)- is situated between the first and 

 third phalanges, its direction corresponding to that of the first phalaiLx. It is 

 flattened from before backward, and its width is greater than its height. It may 

 be described as possessing four surfaces. 



The upper or proximal surface presents two glenoid cavities separated by a 

 low ridge, and articulates with the first phalanx. The middle of the anterior border 

 is elevated and roughened in front for the attachment of the anterior or common 

 extensor tendon. The posterior border is thick and overhanging; in the fresh 

 state its middle part is covered with cartilage, over which the deep flexor tendon 

 passes. On either side there is an eminence, to which the lateral ligament and the 

 superficial flexor tendon are attached. 



The inferior or distal surface is trochlear, and articulates with the third phal- 

 anx and third sesamoid bone. It resembles somewhat the trochlea of the first 

 phalanx, but is more extensive and encroaches more on the anterior and posterior 

 surfaces. 



The anterior or dorsal surface is convex from side to side and smooth in its 



1 It is also called the large pastern bone or os suffraginis. 

 ^ This bone is also called the small pastern bone or os coronse. 



