THE TARSUS 



199 



The Cuboid 



The cuboid (figs. 242, 243, 244), irregularly cubical in shape, is placed on the 

 lateral aspect of the foot, forming a continuous line with the calcaneus and the 

 fourth and fifth metatarsals. 



Its posterior surface is somewhat quadrangular with rounded angles and presents a saddle- 

 shaped articular surface for the calcaneus. Its lower and medial angle is somewhat prolonged 

 backward beneath the sustentaculum tali (calcaneal process of the cuboid), an arrangement to 

 oppose the upward or outward movement of the bone. This process occasionally terminates 



Fig. 242. — The Left Cuboid. (Medial view.) 



For third cuneiform 



For calcaneus 



For fourth metatarsal 



Groove for tendon of the peroneus 

 longus 



in a rounded facet which plays on the head of the talus lateral to the facet for the calcaneo 

 navicular ligament. The anterior surface is smaller and divided by a vertical ridge into two 

 articular facets, a lateral for the base of the fifth, and a medial for the base of the fourth meta- 

 tarsal. The superior surface is rough, non-articular, and directed obliquely upward. The 

 inferior surface presents a prominent ridge for the attachment of the long plantar (calcaneo- 

 cuboid) ligament, in front of which is a deep groove — the peroneal groove — running obliquely 

 forward and medially and lodging the tendon of the peroneus longus. The ridge terminates 

 laterally in an eminence, the tuberosity of the cuboid, on which there is usually a facet for a 

 sesamoid bone of the tendon contained in the groove. The part of the surface behind the ridge 

 is rough for the attachment of the plantar (short) calcaneo-cuboid ligament, a slip of the tibialis 

 posterior, and a few fibres of the flexor hallucis brevis. 



Fig. 24.3. — The Left Cuboid. (Medial view.) 



For third cuneiform — /"-^ 

 For calcaneus — 

 For navicular (occasional) 



/'-' >J 



Groove for tendon of the peroneus ^^=^<^.^C_^ ^' 



longus 



The medial surface presents, near its middle and upper part, an oval facet for articulation 

 with the third cuneiform bone (fig. 242) ; behind this, a second facet for the navicular is fre- 

 quently seen (fig. 243). Generally the two facets are confluent and then form an elliptical 

 surface (fig. 244). The remainder of this surface is rough for the attachment of strong inter- 

 osseous ligaments. 



The lateral surface, the smallest and narrowest of all the surfaces, presents a deep notch 

 which leads into the peroneal groove. 



Articulations. — With the calcaneus behind, fourth and fifth metatarsals in front, third 

 cuneiform and frequently the navicular on the medial side; occasionally also the talus. 



Fig. 244. — The Left Cuboid. (Medial view.) 



For third cuneiform 

 For navicular 



Ossification. — The cuboid is ossified from a single nucleus which appears about the time 

 of birth. 



Accessory tarsal elements. — As in the carpus, a number of additional elements may 

 occur in the tarsus. The most frequent of these is the os trigonum, which has already been 

 noticed. Next in frequency is an additional first cuneiform, resulting from the ossification of 

 the plantar half of that bone independently of the dorsal half, so that the bone is represented 

 by a plantar and a dorsal first cuneiform. Other additional elements may occasionally occur 

 at the upper posterior angle of the sustentaculum tali; at the anterior superior angle of the cal- 

 caneus, between that bone and the navicular; in the angle between the first cuneiform and the 

 first and second metatarsals; and in the fibular angle between the fifth metatarsal and the 

 cuboid (os Vesalianum). 



