PELVIC LIMBS 715 



point where the upper third of the bone joins the lower two-thirds. 

 The other branch passes downward to join the true internal border in 

 the lower fifth of the bone. 



(1) The internal part of the posterior surface is not only separated 

 from the external part by this arrangement, but is itself divided into 

 (a) an upper and (b) a lower area. 



(a) The upper area is about one-third as long as the bone, and is 

 triangular, the apex of the triangle pointing downward and forward. 

 It is transversely strongly convex above, where it faces backward and 

 inward ; it is slightly convex below, where it faces inward. It is 

 occupied by the insertion of the popliteus muscle. 



(b) The lower area of the inner part of the posterior surface has 

 the shape of a long, narrow spindle, flattened or only slightly convex 

 transversely and facing directly inward. The greater part is occupied 

 by the area of origin of the flexor longus digitorum muscle. 



(2) The outer part of the posterior surface has almost parallel 

 lateral borders, but it is slightly narrower above than below. It is 

 also divided into two areas by an oblique line which arises above in 

 the external border of the bone, just under the articular facet for the 

 fibula, and runs downward and inward to join, at the middle of the 

 bone, the outer branch of the longitudinal ridge above described. 

 The upper area is triangular and concave in both directions above, but 

 nearly flat below. It is deeply depressed at its upper inner angle, 

 where are found a number of foramina for blood-vessels ; it affords 

 origin through its entire extent to the tibialis posticus muscle. The 

 lower area of this part of the posterior surface is in the form of a long 

 triangle, the apex whereof is at the upper end of the external border. 

 It gives origin to part of the flexor longus hallucis. In its upper part 

 it is nearly flat from side to side and concave from above downward ; 

 in the lower part it is convex in both directions. On this surface, at 

 about the junction of the upper third of the bone with the lower two- 

 thirds and midway between the oblique line and the outer border, is the 

 main nutrient foramen of the tibia, which pierces the bone obliquely 

 downward. 



The external surface of the shaft (Fig. 544) lies between the 

 anterior and external borders, below the prominent lower edge of the 

 external surface of the outer tuberosity and continuous below with the 

 anterior surface of the lower extremity. For its upper third it occu- 



