THE TIBIA, OR SHIN BONE 



235 



The Distal or Lower Extremity. The lower extremity, much smaller than 

 the upper, presents five surfaces; it is prolonged downward, on its inner side, 

 to a strong process, the internal malleolus (maVeolus medialis). 



The inferior surface (fades articularis inferior) of the bone is quadrilateral, 

 and smooth for articulation \vith the astragalus. This surface is concave from 

 oefore backward, and broader in front than behind. It is traversed from 

 oefore backward by a slight elevation, separating two lateral depressions. It is 

 narrow internally, where the articular surface becomes continuous with that on 

 ;he inner malleolus. 



The anterior surface of the lower extremity is smooth and rounded above, and 

 covered by the tendons of the Extensor muscles of the toes; its lower margin 

 ^resents a rough transverse depression, for the attachment of the anterior ligament 

 of the ankle-joint. 



The posterior surface presents a superficial groove directed obliquely downward 

 and inward, continuous with a similar groove on the posterior surface of the as- 

 xagalus, and serving for the passage of the tendon of the Flexor longus hallucis. 



The external surface presents a triangular rough depression for the attachment 

 of the interior interosseous ligament, connecting it with the fibula; the lower part 

 of this depression, the incisura fibularis, is smooth, covered with cartilage in the 

 recent state, and articulates with the fibula. This surface is bounded by two 

 prominent borders, continuous above with the interosseous ridge; they afford 

 attachment to the anterior inferior and posterior inferior tibiofibular ligaments. 



The internal surface of the lower extremity is prolonged downward to form a 

 strong pyramidal process, flattened from without inward the internal malleolus 

 (malleolus medialis). The inner surface of this process is convex and subcu- 

 ;aneous; its outer surface is smooth and 



Upper extremity. 



Appears shortly 

 after birth. 



.Joins shaft about 

 20ih year. 



slightly concave, and articulates with the 

 astragalus; its anterior border is rough, 

 'or the attachment of the anterior fibres 

 of the internal lateral or deltoid ligament ; 

 ts posterior border presents a broad and 

 deep groove (sulcus malleolaris), directed 

 obliquely downward and inward, which 

 s occasionally double; this groove trans- 

 mits the tendons of the Tibialis posticus 

 and Flexor longus digitorum muscles. 

 The summit of the internal malleolus 

 s marked by a rough depression behind, 

 'or the attachment of the internal lateral 

 igaments of the ankle-joint. 



Structure. Like that of the other long bones. 

 \t the junction of the middle and lower third, 

 svhere the bone is smallest, the wall of the shaft 

 s thicker than in other parts, in order to com- 

 >ensate for the smallness of the calibre of the 

 x)ne. 



Development. From three centres (Fig. 



18), one for the shaft and one for each extrem- 

 ty. Ossification commences in the centre of the 

 shaft about the seventh week, and gradually ex- 

 ends toward either extremity. The centre for the 

 upper epiphysis appears before or shortly after 



nrth; it is flattened in form, and has a thin, tongue-shaped process in front which forms the 

 ubercle. That for the lower epiphysis appears in the second year. The lower epiphysis 

 oins the shaft at about the eighteenth, and the upper one about the twentieth year. Two 

 additional centres occasionally exist one for the tongue-shaped process of the upper epiphysis, 

 which forms the tubercle, and one for the inner malleolus. 



Appears at 2nd_ 

 year. 



Joins shaft about 

 18th year. 



Lower extremity- 



FIG. 188. Plan of the development of the tibia. 

 From three centres. 



