THE TIBIA. 



195 



facet for the fibula obliquely downward, to the internal border, at the junction of its 

 upper arid middle thirds. It marks the lower limit for the insertion of the Popliteus 

 muscle, and serves for the attachment of the popliteal fascia and part of the Soleus. 

 Flexor longus digitorum, and Tibialis posticus muscles ; the triangular concave sur- 

 face, above and to the inner side of this line, gives attachment to the Popliteus 

 muscle. The middle third of the posterior surface is divided by a vertical ridge 

 into two lateral halves : the ridge is well marked at its commencement at the 

 oblique line, but becomes gradually indistinct below ; the inner and broader half 

 gives attachment to the Flexor longus digitorum, the outer and narrower to part of, 

 the Tibialis posticus. The remaining part of the bone presents a smooth surface 

 covered by the Tibialis posticus, Flexor longus digitorum, arid Flexor longus hal- 

 lucis muscles. Immediately below the oblique line is the medullary foramen, which 

 is large and directed obliquely downward. 



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. 

 The inferior surface of the bone is quadrilateral, and smooth for articulation with the 

 astragalus. This surface is concave from before backward, and broader in front than 

 behind. It is traversed from before backward by a slight elevation, separating two 

 lateral depressions. It is narrow internally, where the articular surface becomes con- 

 tinuous with that on the inner malleolus. The anterior surface of the lower extrem- 

 ity is smooth and rounded above, and covered by the tendons of the Extensor 

 muscles of the toes ; its lower margin presents 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 astragalus, 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 inferior interosseous ligament, 

 connecting it with the fibula ; the lower part of this depression is smooth, covered 

 with cartilage in the recent state, and 

 articulates with the fibula. This surface 

 is bounded by two prominent borders, con- 

 tinuous above with the interosseous ridge ; 

 they afford attachment to the anterior and 

 posterior inferior tibio-fibular ligaments. 

 The internal surface of the lower extrem- 

 ity is prolonged downward to form a strong 

 pyramidal process, flattened from without 

 inward the internal malleolus. The inner 

 surface of this process is convex and sub- 

 cutaneous ; its outer surface is smooth and 

 slightly concave, and articulates with the 

 astragalus ; its anterior border is rough, 

 for the attachment of the anterior fibres 

 of the internal lateral or Deltoid ligament ; 

 its posterior border presents a broad and 

 deep groove, directed obliquely downward 

 and inward, which is occasionally double : 

 this groove transmits the tendons of the 

 Tibialis posticus and Flexor longus digi- 

 torum muscles. The summit of the in- 

 ternal malleolus is marked by a rough 

 depression behind, for the attachment of 

 the internal lateral ligament of the ankle- 

 joint. 



Structure. Like that of the other long bones. At the junction of the middle 

 and lower third, where the bone is smallest, the wall of the shaft is thicker than 

 in other parts, in order to compensate for the smallness of the calibre of the bone. 



Upper extremity. 



.ippears at 

 birth. 



Appears at 2n&^ 

 year. 



shaft abou: 

 20th year. 





Joins shaft about 

 18th year. 



Lower erlremity. 



FIG. 135. Plan of the development of the tibia. 

 By three centres. 



