THE TARSUS 



195 



foetus at birth the angle averages 35°, whilst in a young orang it measures 45°. In the normal 

 adult talus the articular surface on the tibial side is limited to the body of the bone. In the 

 foetal talus it extends for some distane on to the neck, and sometimes reaches almost as far 

 forward as the navicular facet on the head of the bone. This disposition of the medial malleolar 

 facet is a characteristic feature of the talus in the chimpanzee and the orang. It is related to 

 the inverted position of the foot which is found in the human foetus almost up to the period of 

 birth, and is of interest to the surgeon in connection with some varieties of club-foot. (Shattock 

 and Parker.) 



The Calcaneus 



The calcaneus (or os calcis) (figs. 232, 233) is the largest and strongest bone of 

 the foot. It is of an elongated form, flattened from side to side, and expanded at 

 its posterior extremity, which projects downward and backward to form the heel. 

 It presents six surfaces, superior, inferior lateral, medial, anterior and posterior. 



The superior surface presents in the middle a large, oval, convex, articular facet for the 

 under aspect of the body of the talus. In front of the facet the bone is marked by a deep 



Fig. 232. — The Left Calcaneus. (Dorsal view.) 



Medial process 



Calcaneal groove 



Facet for talus on the sustentaculuic 

 tali 



For talus 



Peroneal tubercle 



depression, the floor of which is rough for the attachment of ligaments, especially the talo- 

 calcaneal, and the origin of the extensor digitorum brevis muscle; when the calcaneus and talus 

 are articulated, this portion of the bone forms the floor of a cavity called the sinus tarsi. Medi- 

 ally, the upper surface of the bone presents a well-marked process, the sustentaculum tali, 

 furnished with an elongated concave facet, occasionally divided into two, for articulation with 

 the under aspect of the head of the talus. The posterior part of the upper surface is non- 

 articular, convex from side to side, and in relation with a mass of fat placed in front of the 

 tendo Achillis. 



The inferior surface is narrow, rough, uneven, and ends posteriorlj^ in two processes: the 

 medial is the larger and broader, the lateral is narrower but prominent. The medial process 

 aifords origin to the abductor hallucis, the flexor digitorum brevis, and the abductor digiti quinti; 

 the last muscle also arises from the lateral process and from the ridge of bone between. The 

 rough surface in front of the tubercles gives attachment to the long plantar ligament (calcaneo- 

 cuboid) and the lateral head of the quadratus plantce. Near its anterior end this surface forms 

 a rounded eminence, the anterior tubercle, from which (as well as from the shallow groove in 

 front) the plantar (short) calcaneo-cuboid ligament arises. (According to the BNA nomen- 

 clature, the medial and lateral processes belong to the tuber calcanei or the posterior extremity 

 of the bone.) 



_ The lateral surface is broad, flat, and slightly convex. It represents near the middle a small 

 eminence for the calcaneo-fibular ligament of the ankle-joint. Below and in front of this is a 

 well-marked tubercle — the trochlear process [processus trochlearis] (or peroneal tubercle), 

 separating two grooves, the upper for the -peroneus brevis and the lower for the peroneus longus. 



. The medial surface is deeply concave, the hollow being increased by the prominent medial 

 process behind and the overhanging sustentaculum tali in front. The latter forms a promi- 

 nence of bone projecting horizontally, concave and articular above, grooved below for the 

 tendon of the flexor hallucis longus, and giving attachment to a slip of the tendon of the tibialis 

 posterior, the inferior calcaneo-navicular ligament, and some fibres of the deltoid ligament of 

 the ankle-joint. The hollow below the process receives the plantar vessels and nerves and its 

 lower part gives attachment to the medial head of the quadratus plantcp. 



