246 THE SKELETON. 



The external surface is narrower than the internal ; its upper two-thirds pre- 

 sent a shallow groove for the attachment of the Tibialis Anticus muscle; its 

 lower third is smooth, convex, curves gradually forwards to the anterior part 

 of the bone, and is covered from within outwards by the tendons of the follow- 

 ing muscles : Tibialis Anticus, Extensor Proprius Pollicis, Extensor Longus 

 Digitorum, Peroneus Tertius. 



The posterior surface (Fig. 177) presents, at its upper part, a prominent ridge, 

 the oblique line of the tibia, which extends from the back part of the articular 

 facet for the fibula, obliquely downwards, to the internal border, at the junction 

 of its upper and middle thirds. It marks the 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 surface, above and to the inner side of this line, gives attach- 

 ment 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 is covered by the Tibialis Posticus, Flexor Longus Digitorum, and 

 Flexor Longus Pollicis muscles. Immediately below the oblique line is the 

 medullary foramen, which is directed obliquely downwards. 



The Lower Extremity, much smaller than the upper, presents five surfaces ; it 

 is prolonged downwards, on its inner side, into a strong process, the internal 

 malleolus. The inferior surface of the bone is quadrilateral, and smooth, for 

 articulation with the astragalus. This surface is narrow internally, where it 

 becomes continuous with the articular surface of the inner malleolus, broader 

 externally, and traversed from before backwards by a slight elevation, sepa- 

 rating two lateral depressions. The anterior surface of the lower extremity is 

 smooth and rounded above, and covered by the tendons of the extensor mus- 

 cles 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 downwards and inwards, con- 

 tinuous with a similar groove on the posterior extremity of the astragalus, and 

 serving for the passage of the tendon of the Flexor Longus Pollicis : the < 

 nal surface presents a triangular rough depression for the attachment of th<3 

 inferior interosseous ligament connecting it with the fibula ; the lower part of 

 this depression is smooth in some bones, covered with cartilage in the recent 

 state, and articulating with the fibula. This surface is bounded by two promi- 

 nent ridges, continuous above with the interosseous ridge ; they afford attach- 

 ment to the anterior and posterior tibio-fibular ligaments. The internal surface 

 of the lower extremity is prolonged downwards to form a strong pyramidal 

 process, flattened from without inwards, the inner malleolus. The inner surface 

 of this process is convex and subcutaneous; its outer surface, smooth and slightly 

 concave, deepens the articular surface for the astragalus ; its anterior border is 

 rough, for the attachment of ligamentous fibres ; its posterior border presents a 

 broad and deep groove, directed obliquely downwards and inwards, which is 

 occasionally double: this groove transmits the tendons of the Tibialis Posticus 

 and Flexor Longus Digitorum muscles. The summit of the internal 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. 



Development. By three centres (Fig. 178): one for the shaft, and one for each 

 extremity. Ossification commences in the centre of the shaft about the same 

 time as in the femur, the fifth week, and gradually extends towards either 

 extremity. The centre for the upper epiphysis appears at birth; it is flat- 

 tened in form, and has a thin tongue-shaped process in front, which forms 

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



