CHAP. IV.] THE SKELETON OF THE LIMBS. 113 



of the peroneus longus muscle passes in front of the external 

 malleolus. 



Thus, altogether wo have at the lower end of the cat's leg, a 

 median horizontal surface for the tarsus, with two other articular 

 surfticos, at right angles with the horizontal one, and formed by the 

 surfaces of the malleoli. 



§ 17. The TARSUS consists of seven bones (none of which can be 

 called "long bones"), namely the astragalus, calcaneum, cnboides, 

 naviculare, and three cuneiform bones. All these are so firmly 

 connected by ligamentous fibres which envelope them, that very 

 little mobility is possible, though there may be a very 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-Hke joint by which the tarsus articulates with the bones 

 of the leg. 



The astragalus receives the weight of the trunk from the tibia, 

 and is a short irregularly shaped bone (Fig. 73, as), with a " body, 

 neck, and head." 



In its natural position in walking, when the foot rests on the 

 ground only by its toes (the heel being raised high up) the upper 

 surface of the body of the astragalus (by which it articulates with 

 the under surface of the shaft of the tibia) looks forwards as well as 

 upwards. Two other articular surfaces, almost at right angles with 

 the former, join the two malleoli respectively, that for the internal 

 malleolus being much the larger. The posterior surface of the body 

 is grooved for the tendon of the flexor longus hallucis muscle. The 

 anterior part of the bone is prolonged forwards as its neck, ending 

 in a rounded, convex, articular surface (the head), which fits into 

 the hinder surface of the naviculare {n). The anterior part of the 

 dorsum of the bone presents a concavity, which gives origin to the 

 extensor brevis digitorum muscle, and also affords attachment to a 

 tendinous loop through which pass the tendons of the extensor 

 longus digitorum muscle. 



The calcaneum (or os calcis) is the bone of the heel (oc), and is by 

 far the largest bone of the tarsus. It is rather more than twice as 

 long as broad, and is somewhat expanded at its hinder end (called 

 its tuberosity), which is vertically grooved to allow the tendon of 

 the Plantaris muscle to play over it (Fig. 72, oc^) . The calcaneum 

 articulates with the astragalus above and with the cuboid in front. 

 It developes a process on its inner side {x) to support part of the 

 last-named bone, and another smaller process {oc ") on its outer side 

 just before joining the cuboids. 



The naviculare (or scaphoid of the foot) is, on its dorsal surface, 

 wider than long (Fig. 73, A). Behind, it is deeply concave for the 

 reception of the head of the astragalus. In front it presents three 

 surfaces for articulation with the three cuneiform bones. That for 

 the middle one is convex. 



Its inner margin sends backwards a long process, called its 



I 



