THE TARSUS 191 



is the interval between the medial and lateral crests, and is somewhat twisted so 

 as to look backward above and medially below. It serves, in its upper third, for 

 the origin of the soleus, and in its lower two-thirds for the flexor hallucis longus. 

 Near the middle of the surface is the medullary foramen, directed downward 

 toward the ankle. The lateral surface, situated between the anterior and lateral 

 crests, is also somewhat twisted, looking laterally above and backward below, 

 where it is continuous with the groove on the back of the lateral malleolus. 

 The surface is often deeply grooved and is occupied by the peroneus longus in the 

 upper two-thirds and by the peroneus hrevis in the lower two-thirds. 



The lateral malleolus or lower extremity is pyramidal in form, somewhat 

 flattened from side to side, and joined by its base to the shaft. It is longer, more 

 prominent, and descends lower than the medial malleolus. Its lateral surface is 

 convex, subcutaneous, and continuous with the triangular subcutaneous surface 

 on the shaft, immediately above. The medial surface is divided into an anterior 

 and upper area [facies articularis malleoli], triangular in outline and convex from 

 above downward for articulation with the lateral side of the talus (astragalus) , and 

 a lower and posterior excavated area, the digital fossa, in which are attached the 

 transverse inferior tibio-fibular ligament and the posterior talo-fibular (posterior 

 fasciculus of the external lateral) ligament of the ankle. The anterior border is 

 rough and gives attachment to the anterior talo-fibular (anterior fasciculus of the 

 external lateral) hgament of the ankle, and the anterior inferior tibio-fibular liga- 

 ment. The posterior border is grooved for the peronei tendons, and near its 

 upper part gives attachment to the posterior inferior tibio-fibular ligament. The 

 apex or summit of the process affords attachment to the calcaneo-fibular (middle 

 fasciculus of the external lateral) ligament of the ankle. 



Blood -supply. — The shaft of the fibula receives its nutrient artery from the peroneal 

 branch of the posterior tibial. The head is nourished by branches from the inferior lateral 

 articular branch of the popliteal artery, and the lateral malleolus is supplied mainly by the 

 peroneal, and its perforating and malleolar branches. 



Ossification. — -The shaft of the fibula commences to ossify in the eighth week of intra- 

 uterine life. A nucleus appears for the lower extremity in the second year, and one in the upper 

 extremity during the fourth or fifth year. The lower extremity fuses with the shaft about 

 twenty, but the upper extremity remains separate until the twenty-second year or even later. 



It is interesting, in connection with the times of appearance of the two epiphj^ses of the 

 fibula, to note that the ossification of the lower epiphysis is contrary to the general rule — viz., 

 that the epiphysis toward which the nutrient artery is directed is the last to undergo ossification. 

 This is perhaps explained by the rudimentary nature of the upper extremity. In birds the head 

 of the bone is large and enters into the formation of the knee-joint; and in human embryos, 

 during the second month, the fibula is quite close up to the femur. 



The human fibula is characterised by the length of its malleolus, for in no other vertebrate 

 does this process descend so far below the level of the tibial malleolus. On the other hand, in 

 the majority of mammals the tibial descends to a lower level than the fibular nialleolus. In 

 the human embryo of the third month, the lateral is equal in length to the medial malleolus. 

 At the fifth month the lateral malleolus exceeds the medial by 1.5 mm.; at birth, the lateral 

 malleolus is still longer; and by the second year it assumes its adult proportion. 



THE TARSUS 



The tarsal bones [ossa tarsi] (figs. 228, 229) are grouped in two rows:— a 

 proximal row, consisting of the talus and calcaneus, and a distal row, consisting 

 of four bones which, enumerated from tibial side, are the first, second, and third 

 cuneiform bones and the cuboid. Interposed between the two rows on the tibial 

 side of the foot is a single bone, the navicular ; on the fibular side the proximal and 

 distal rows come into contact. 



Compared with the carpus, the tarsal bones present fewer common characters, 

 and greater diversity of size and form, in consequence of the modifications for sup- 

 porting the weight of the trunk. On each, however, six surfaces can generally be 

 recognised, articular when in contact with neighbouring bones, elsewhere sub- 

 cutaneous or rough for the attachment of ligaments. As regards ossification, they 

 correspond in the main with that of the bones of the carpus. Each tarsal bone is 

 ossified from a single centre, but the calcaneus has, in addition, an epiphysis for a 

 large part of its posterior extremity, and the talus, an occasional centre for the os 

 trigonum. 



