742 



MAMMALIAN ANATOMY 



FIG. 559. 



vertical longitudinal plane as the whole bone, but is directed from the 

 proximal end distally and to the fibular side. The centre of the distal 



end of the trochlea is situated at the beginning 

 of the fibular margin of the neck ; the tibial 

 part of the trochlea is continued on the neck 

 as its dorsal surface. This surface of the neck 

 is convex from side to side and concave from 

 its distal to its proximal end, and is perforated 

 by a number of small foramina ; it gives origin 

 to fibres of the extensor brevis digitorum mus- 

 cle. It is continued distally as the dorsal sur- 

 face of the head. 



The fibular or external surface of the body 

 (Fig. 566) is crescentic, and is limited on the 

 proximal and dorsal sides by the convex line 

 of the trochlea, which arches from the proximal 

 end, distally and to the plantar side, and forms 

 an angle with the distal margin, which is trun- 

 cate and directed proximally and to the plantar 

 side. The plantar margin is prominent in its 

 distal part, but becomes obscure proximal to the 

 middle and is lost in the rough pit for the in- 

 sertion of the strong ligament from the inner 



o O 



surface of the external malleolus. Almost the 

 entire surface is occupied by a curved, pyri- 

 form, articular surface for the external malle- 

 olus. The broad part is distal ; the narrow part 

 is proximal. The space at the proximal end 

 not occupied by this articular surface is the rough pit already men- 

 tioned. The surface is nearly flat, slightly concave at the distal end, 

 and faces toward the fibular side and slightly proximally. A small, 

 oblong fibular surface is shown on the side of the neck and head ; it is 

 concave and rough. 



The proximal or posterior surface (Fig. 563) is the continuation 

 of the trochlea toward the plantar side. It is triangular ; the apex of 

 the triangle is plantar ; the sides are prolongations of the sides of the 

 trochlea. It is directed obliquely to the plantar and tibial sides, and is 

 deeply grooved for the passage of the flexor muscles of the foot. The 



POSITION OF ASTRAGALUS. 



