184 ELEMENTARY ANATOMY. [LESS. 



duced into a sharp crest in front, on the internal side of 

 which crest the bone is convex, and concave on its outer side. 

 At the upper end of the front of the shaft is a prominence 

 called the tuberosity, or tubercle. 



The lower end of the bone is much smaller than the 

 upper. Its lower border has a single groove behind for the 

 tendons of the tibialis posticus and flexor longus digitorum 

 muscles. Its outer surface is concave for the reception of the 

 fibula. Its inner margin is produced downwards into a strongly 

 marked triangular process, called the internal malleolus. 

 This forms the bony projection on the inside of the ankle, 

 and articulates with the inner side of the tarsus. Another 

 articular surface, concave and quadrilateral, is situated on 

 the under-surface of the lower end of the tibia, and articulates 

 with the summit of the tarsus. The hinder margin of this 

 articular surface descends below its anterior border. 



5. The FIBULA is the slenderest bone, in proportion to its 

 length, in the body, and extends on the outer side of the leg 

 from near the knee down to the ankle. 



Its upper extremity is slightly enlarged into what is called 

 the head, which articulates with the outer side of the head of 

 the tibia, and gives insertion to the external lateral ligament. 

 It does not mount upwards so high as does the tibia. 



The shaft of the bone is irregularly triangular in section. 

 Its lower end is expanded into what is called the external 

 malleolus, which forms the bony projection on the outer side 

 of the ankle and articulates with the outer side of the tarsus. 



This malleolus projects downwards considerably further 

 than does the internal malleolus. On its inner side it articu- 

 lates with the tibia. 



Thus we have in man a horizontal articular surface for the 

 tarsus, formed by the lower end of the tibia, and two other 

 articular surfaces, at right angles with it, formed by the sur- 

 faces of the malleoli. 



6. The TARSUS consists of seven bones (none of which 

 can be called "long bones"), namely, the astragalus, calca- 

 neum, cuboides, naviculare, and three cuneiform bones. All 

 these are so firmly connected by ligamentous fibres which en- 

 velop them, that very little mobility is possible, though there 

 may be a slight rotation of the distal tarsal bones upon the 

 proximal ones, that is, upon the astragalus and calcaneum. 



The movement of the foot on the leg, however, takes place 

 entirely by the hinge-like joint by which the tarsus articu- 

 lates with the bones of the leg. 



