310 THE SKELETON. 



is more solidly constructed, and its component parts are less movable on each other 

 than in the hand. This is especially the case with the great toe, which has to 

 assist in supporting the body, and is therefore constructed with greater solidity ; it 

 lies parallel with the other toes, and has a very limited degree of mobility, whereas 

 the thumb, which is occupied in numerous and varied movements, is constructed 

 in such a manner as to permit of great mobility. Its metacarpal bone is directed 

 away from the others, so as to form an acute angle with the second, and it enjoys- 

 a considerable range of motion at its articulation with the carpus. The foot is 

 placed at right angles to the leg a position which is almost peculiar to man, and 

 has relation to the erect position which he maintains. In order to allow of its 

 supporting the weight of the whole body in this position with the least expenditure 

 of material, it is constructed in the form of an arch. This arch is not, however, 

 made up of two equal limbs. The hinder one, which is made up of the os calcis 

 and the posterior part of the astragalus, is about half the length of the anterior 

 limb, and measures about three inches. The anterior limb consists of the rest of 

 the tarsal and the metatarsal bones, and measures about six inches. It may be 

 said to consist of two parts, an inner segment made up of the head of the astragalus, 

 the navicular, the three cuneiform, and the three inner metatarsal bones ; and an 

 outer segment composed of the cuboid and the two outer metatarsal bones. The 

 summit of the arch is at the superior articular surface of the astragalus ; and its- 

 two extremities that is to say, the two points on which the arch rests in standing 

 are the tubercles on the under surface of the os calcis posteriorly, and the heads of 

 the metatarsal bones anteriorly. The weakest part of the arch is the joint between 

 the astragalus and scaphoid, and here it is more liable to yield in those who are 

 overweighted, and in those in whom the ligaments which complete and preserve 

 the arch are relaxed. This weak point in the arch is braced on its concave surface 

 by the inferior calcaneo-navicular ligament, which is more elastic than most other 

 ligaments, and thus allows the arch to yield from jars or shocks applied to the 

 anterior portion of the foot and quickly restores it to its pristine condition. This 

 ligament is supported on its under surface by the tendon of the Tibialis posticus 

 muscle, which is spread out into a fan-shaped insertion, and prevents undue 

 tension of the ligament or such an amount of stretching as would permanently 

 elongate it. 



In addition to this longitudinal arch the foot presents a transverse arch, at the 

 anterior part of the tarsus and hinder part of the metatarsus. This, however, can 

 scarcely be described as a true arch, but presents more the character of a half-dome. 

 The inner border of the central portion of the longitudinal arch is elevated from 

 the ground, and from this point the bones arch over to the outer border, which is 

 in contact with the ground, and, assisted by the longitudinal arch, produce a sort of 

 rounded niche on the inner side of the foot, which gives the appearance of a 

 transverse as well as a longitudinal arch. 



The arch of the foot, from the point of the heel to the toes, is not quite straight, 

 but is directed a little outward, so that the inner border is a little convex and the 

 outer border concave. This disposition of the bones becomes more marked when 

 the longitudinal arch of the foot is lost, as in the disease known under the name 

 of "flat-foot." 



Surface Form. On the dorsum of the foot the individual bones are not to be distinguished 

 with the exception of the head of the astragalus, which forms a rounded projection in front of 

 the ankle-joint when the foot is forcibly extended. The whole surface forms a smooth convex 

 outline, the summit of which is the ridge formed by the head of the astragalus, the navicular, 

 the middle cuneiform, and the second metatarsal bones ; from this it gradually inclines outward 

 and more rapidly inward. On the inner side of the foot, the internal tuberosity of the os calcis 

 and the ridge separating the inner from the posterior surface of the bone may be felt most pos- 

 teriorly. In front of this, and below the internal malleolus, may be felt the projection of the 

 sustentaculum tali. Passing forward is the well-marked tuberosity of the navicular bone, situ- 

 ated about an inch or an inch and a quarter in front of the internal malleolus. Further toward 

 the front, the ridge formed by the base of the first metatarsal bone can be obscurely felt, and 

 from this the shaft of the bone can be traced to the expanded head articulating with the base 

 of the first phalanx of the great toe. Immediately beneath the base of this phalanx, the 



