730 MAMMALIAN ANATOMY 



surfaces, and gives attachment to fascia separating the peroneal and 

 flexor muscles. 



The posterior-internal border is short and inconspicuous. It 

 arises near the posterior-external border, but from the inner side of 

 the posterior eminence of the head ; it runs obliquely downward and 

 forward and joins the anterior-internal border in the upper fifth of 

 the bone. From its course it is also known as the oblique line of the 

 fibula. It separates the small superior triangular internal surface from 

 the larger posterior surface. 



The anterior surface lies between the anterior-internal and anterior- 

 external borders. It is very narrow above, but gradually becomes 

 wider, and at the middle of the bone reaches its full width, which it 

 retains almost to the end, where it again becomes pointed. It is 

 rounded above, but for most of its length is flattened, and below 

 slightly concave transversely. In its upper part it faces outward 

 and forward ; at the lower end it twists so as to face almost directly 

 outward. For most of its extent it affords attachment to the peroneus 

 brevis. 



The external surface (Fig. 554) lies between the anterior-external 

 and posterior-external borders. It is a narrow strip of equal width 

 throughout its extent, convex transversely and gently sinuous verti- 

 cally. It faces outward and backward, and affords attachment to the 

 soleus and peroneus tertius. 



The internal surface, included between the anterior-internal and 

 posterior-internal borders, is limited to a very small triangle at the 

 upper inner side of the bone. It faces inward and forward, is faintly 

 concave, and is roughened to give origin to the tibialis anticus muscle. 



The posterior surface (Fig. 555) is the largest and best defined, 

 and lies between the posterior-internal and anterior-internal borders 

 on one side and the posterior-external border on the other side. It 

 is narrow in its upper fifth, below which it comprises the entire width 

 of the bone. It is gently rounded transversely above, but for the 

 most part is flat or faintly concave. It bends slightly toward the tibia 

 at the middle, and faces backward and inward above and directly in- 

 ward below. At about the lower limit of the upper fourth, near the 

 anterior internal border, at the end of a long groove, is the nutrient 

 foramen, which is directed downward. The surface gives attachment 

 through its entire length to the flexor longus hallucis muscle. 



