172 Anatomy of Skeleton 



pressure, and about an inch in front of this the tubercle of the scaphoid is easily found. 

 Pass the fingers carefully along the inner side in front of this, and it is possible to 

 distinguish the bony masses of the inner cuneiform and first metatarsal from each other. 



On the other side the external malleolus and the styloid prominence of the fifth 

 metatarsal, below and in front of it, are the only points that can be found : the edge of 

 the tarsus between these lies so deep and is so covered by soft parts that it cannot be 

 certainly felt. 



But the general level of the mid-tarsal joint can be placed on the outer side as 

 half-way between these points, while on the inner side it lies, of course, just behind the 

 scaphoid tubercle. This is an important joint, at which a large part of the free move- 

 ments of the foot take place. 



In the metatarsal region it is enough to point out at present that there is a trans- 

 verse arch continuous with that of the tarsus but not so deep : attention can also be 

 called to the presence of a couple of sesamoid bones on the plantar surface of the head 

 of the first metatarsal, which are the largest in the body, after the patella, and which 

 account for the marked grooves present on this surface of the articular head. 



FIG. 141. To illustrate methods by which an arch composed of segments 

 may be held up : i, by ties between the segments in the concavity ; 

 2, by connecting the supporting pillars ; and 3, by the direct support 

 of a strap passing under the highest part of the arch. In the lower arch 

 the first two methods are much less effective and the connections must 

 therefore be proportionately stronger, but the direct action of 3 is not 

 lessened, though more strain must be thrown on it owing to the lessened 

 effectiveness of the other supports. 



We can now examine the construction of the arches present in the foot and obtain 

 a general idea of the mechanical factors that support them. 



There is a longitudinal arch in the foot, as well as a transverse concavity, and it is 

 convenient to deal with the former first. 



If the skeleton of the foot is considered, it is evident that the bones which con- 

 stitute the longitudinal arch can be divided into two groups, inner and outer, resting 

 on a common pillar, the os calcis, at their posterior ends. The inner arch is higher, and 

 is made by the astragalus (resting on os calcis) , scaphoid, cuneiforms, and three inner 

 metatarsals, while the outer comprises os calcis, cuboid, and two outer metatarsals. 

 The two sets of bones are in a sense distinct, for the articulations between them permit 

 of sliding movements only, so that one arch could be momentarily increased or lessened 

 without affecting the other. As there is no interlocking between the arches in front 

 of the calcaneum, each must depend on its own mechanism for support, and such 

 mechanism will vary with the conditions of the arch. 



The means used to maintain each arch can be divided into three main classes. 

 If an arch, composed of several segments, as in Fig. 141, requires support, this can be 

 secured in three ways : (i) The segments can be tied together c*i the side of the con- 

 cavity ; ties on the convex side would have no effect. (2) The supporting pillars 

 can be tied together. (3) A strap can be run under the highest part of the arch, fastened 

 to structures away from the arch. 



