298 THE SKELETON. 



The postero-internal border, sometimes called the oblique line, commences above 

 at the inner side of the head, and terminates by becoming continuous with the 

 antero-internal border or interosseous ridge at the lower fourth of the bone. It is 

 well marked and prominent at the upper and middle parts of the bone. It gives 

 attachment to an aponeurosis which separates the Tibialis posticus from the Soleus 

 above and the Flexor longus hallucis below. 



The anterior surface is the interval between the antero-external and antero- 

 internal borders. It is extremely narrow and flat in the upper third of its extent ; 

 broader and grooved longitudinally in its lower third ; it serves for the attachment 

 of three muscles, the Extensor longus digitorum, Peroneus tertius, and Extensor 

 proprius hallucis. 



The external surface is the space between the antero-external and postero- 

 external borders. It is much broader than the preceding, and often deeply grooved, 

 is directed outward in the upper two-thirds of its course, backward in the lower 

 third, where it is continuous with the posterior border of the external malleolus. 

 This surface is completely occupied by the Peroneus longus and brevis muscles. 



The internal surface is the interval included between the antero-internal and 

 the postero-internal borders. It is directed inward, and is grooved for the attach- 

 ment of the Tibialis posticus muscle. 



The posterior surface is the space included between the postero-external and 

 the postero-internal borders ; it is continuous below with the rough triangular 

 surface above the articular facet of the outer malleolus ; it is directed backward 

 above, backward and inward at its middle, directly inward below. Its upper 

 third is rough, for the attachment of the Soleus muscle ; its lower part presents 

 a triangular rough surface, connected to the tibia by a strong interosseous ligament, 

 and between these two points the entire surface is covered by the fibres of origin 

 of the Flexor longus hallucis muscle. At about the middle of this surface is the 

 nutrient foramen, which is directed downward. 



The Lower Extremity, or external malleolus, is of a pyramidal form, somewhat 

 flattened from without inward, and is longer, and descends lower than the internal 

 malleolus. Its external surface is convex, subcutaneous, and continuous with the 

 triangular (also subcutaneous) surface on the outer side of the shaft. The internal 

 surface presents in front a smooth triangular facet, broader above than below, and 

 convex from above downward, which articulates with a corresponding surface on 

 the outer side of the astragalus. Behind and beneath the articular surface is a 

 rough depression which gives attachment to the posterior fasciculus of the external 

 lateral ligament of the ankle. The anterior border is thick and rough, and marked 

 below by a depression for the attachment of the anterior fasciculus of the external 

 lateral ligament. The posterior border is broad and marked by a shallow groove, 

 for the passage of the tendons of the Peroneus longus and brevis muscles. The 

 summit is rounded, and gives attachment to the middle fasciculus of the external 

 lateral ligament. 



In order to distinguish the side to which the bone belongs, hold it with the 

 lower extremity downward and the broad groove for the Peronei tendons back- 

 ward i. e. toward the holder: the triangular subcutaneous surface will then be 

 directed to the side to which the bone belongs. 



Articulations. With two bones : the tibia and astragalus. 



Development. By three centres (Fig. 223) : one for the shaft, and one for 

 each extremity. Ossification commences in the shaft about the eighth week of 

 foetal life, a little later than in the tibia, and extends gradually toward the 

 extremities. At birth both ends are cartilaginous. Ossification commences in 

 the lower end in the second year, and in the upper one about the fourth year. 

 The lower epiphysis, the first in which ossification commences, becomes united to 

 the shaft about the twentieth year; the upper epiphysis joins about the twenty- 

 fifth year. Ossification appearing first in the lower epiphysis is contrary to the 

 rule which prevails with regard to the commencement of ossification in epiphyses 

 viz. that that epiphysis toward which the nutrient artery is directed commences 



