446 HUMAN ANATOMY. 



border of the head of the astragalus. The tendon of the tibiahs posticus directly 

 beneath and internal to the joint adds to its security. 



The motions of the two subastragaloid joints must, of course, be considered 

 together. They are resolved into turning on an oblique axis running through the 

 interosseous ligament, somewhat internal to its middle, downward with something 

 of a backward and inward inclination. Rotating on this, the posterior concave 

 articular surface of the astragalus twists with a screw motion on the opposed surface 

 of the calcaneum. As the back of the astragalus moves upward and outward, the 

 head passes downward and inward in the socket. This movement is stopped by the 

 front of the posterior articular surface of the astragalus catching in the hollow at the 

 front of the convex surface of the calcaneum that it plays on. This is a most efficient 

 device for locking the joint. The opposite motion is stopped by the inner posterior 

 tubercle of the astragalus striking the back of the sustentaculum tali. In the anterior 

 joint there is also to be considered the motion between the head of the astragalus 

 and the scaphoid. The strong interosseous and inferior calcaneo-scaphoid ligaments 

 do not allow much displacement of the scaphoid, but it seems that it can travel for 

 a short distance up or down and in or out, and can therefore be slightly circum- 

 ducted ; the chief motion, however, is one of rotation on the above-mentioned axis 

 through the astragalus. Variations in the slant of the posterior articular surface of 

 the OS calcis must, of course, modify the position of the axis. 



THE CALCANEO-CUBOID JOINT. 



The calcaneo-cuboid joint (Fig. 458), surrounded by a capsule the inner side 

 of which is formed by the interosseous calcaneo-cubo-scaphoid ligament, is a modi- 

 fication of the saddle-joint. Apart from some indefinite gliding, the nature and 

 amount of which vary in different feet, the chief motion is rotation on an approxi- 

 mately antero-posterior axis running through the joint. It might, perhaps, be more 

 accurately defined as a screw motion. This movement, however, is very limited. 

 Rotation of the cuboid in a direction that would raise its outer border is checked by 

 the interosseous and dorsal ligaments at its inner side. Rotation in the opposite 

 direction, if not sooner arrested by the ligaments, is effectually checked by the 

 plantar tubercle of the cuboid catching on the overhanging lip of the articular surface 

 of the OS calcis, thus locking the joint. 



THE SCAPHO-CUBO-CUNEIFORM JOINT. 



This articulation is a synovial cavity bounded behind by the scaphoid, extending 

 forward to varying distances between the different bones. Thus, between the first 

 and second cuneiforms it communicates with the joint of the second metatarsal, it is 

 usually bounded by the interosseous ligament between the second and third meta- 

 tarsals, and finally by that between the latter with the cuboid. The motions are 

 very slight in each joint and of no great importance when combined. There is next 

 to no motion of the internal cuneiform of the scaphoid and very little of the second. 

 The external moves more freely, sliding slightly up and down. The interosseous 

 ligaments resist the undue spreading of the transverse arch of the foot. 



THE TARSO-METATARSAL JOINTS. 



That of the first metatarsal bone (Fig. 458) is an independent joint with its 

 own capsule, the interosseous ligament between the internal cuneiform and the 

 second metatarsal shutting it off. The front of the cuneiform is convex from side to 

 side and about plane from above down. Rarely it is subdivided into an upper and a 

 lower compartment. It may be prolonged onto the side of the second metatarsal. 

 An articular facet coated with cartilage is common on the outer side of the first meta- 

 tarsal, but that on the second is indistinct or wanting. It seems that this is simply 

 a bursa in most cases just beyond the joint, but they sometimes communicate. Lat- 

 eral motion with this metatarsal is the most free, and there is a certain sliding up 

 and down. 



