THE CUBOID BONE. 241 
forwards and are attached to the summit of the ridge. The outer aspect of the bone 
is short and rounded, and is formed by the confluence of the superior and inferior sur- 
faces ; it is more or less notched by the peroneal groove which turns round its lower 
edge. The internal surface of the bone is the most extensive ; it is easily recognisable 
on account of the presence of a rounded or oval facet situated near its middle and 
_ close to its upper border. This is for articulation with the outer side of the external 
— cuneiform; in front and behind this the surface is rough for ligaments. Not 
infrequently behind the facet for the external cuneiform there is a small articular 
_ surface for the navicular, as is the case normally in the gorilla, whilst behind and 
_ below, the projecting inferior angle is sometimes provided with a facet, on which 
_ the head of the astragalus rests (Sutton, “ Proc. Anat. Soc.,” Journ. Anat. and Physiol., 
' vol. xxvi. p. 18). The anterior surface is oval or conical in outline; sloping 
— obliquely from within outwards and backwards, it is divided about its middle by a 
, sieht vertical ridge into two parts, the inner of which articulates with the base of 
the fourth metatarsal bone, the outer with that of the fifth. The posterior surface, 
also articular, has a semilunar outline, the convex margin of which corresponds to 
_ the dorsal roundness of the bone. The inferior external angle corresponds to the 
_ tubercle on the outer border of the bone, whilst the inferior internal angle forms a 
_ pointed projection, which is sometimes called the calcanean process. The posterior 
surface articulates with the os calcis by means of a saddle-shaped facet, which is 
convex from side to side, and concave from above downwards. 
Variations.—Blandin has recorded a case of division of the cuboid. 
| The tarsus as a whole may be conveniently described as arranged in two columns; the 
inner, corresponding to the inner border of the foot, comprising the astragalus, navicular, and 
three cuneiforms, and forming a base for the support of the three inner metatarsal bones and 
their phalanges. The outer column, formed by the os caleis and cuboid, supports the fourth 
and fifth metatarsal bones together with their phalanges. The superior surface of the anterior 
ortion of the tarsus determines the side-to-side roundness of the instep, whilst its under surface 
fers arches in both a transverse and longitudinal direction, in which the softer tissues of the 
sole are lodged, and so protected from injury. 
Architecture of the Bones of the Foot.—A longitudinal section through the articulated 
bones of the foot reveals the fact that the cancellous structure of each individual bone is deter- 
mined by the stress to which it is habitually subjected. In this connection it is necessary to 
refer to the arched arrangement of the bones of the foot, a subject which is elsewhere treated in 
detail (see p. 304). The summit of the arch is formed by the astragalus, on which rests the tibia. 
Subjected as the astragalus is to a crushing strain, it is obvious that this load must be distributed 
throughout the arch, of which the os caleis is the posterior pillar, whilst the heads of the meta- 
tarsal bones constitute the anterior pillar. It is found, consequently, that the lamelle of the can- 
cellous tissue of the astragalus are arranged in two directions, which intercross and_ terminate 
below the superior articular surface. Of these fibres, some sweep backwards and downwards 
- towards the posterior calcanean facet, beyond which they are carried in the substance of the os 
calcis in a curved and wavy manner in the direction of the heel, where they terminate ; whilst 
others, curving downwards and forwards from the trochlea of the astragalus, pass through the 
neck to reach the articular surface of the head, through which in like manner they may be 
regarded as passing onwards through the several bones which constitute the anterior part of the 
arch, thus accounting for the longitudinal striation as displayed in the structure of the navi- 
cular, cuneiform and metatarsal bones. In the os ecalcis, in addition to the foregoing arrange- 
ment, another set of curving fibres sweep from back to front of the bone beneath the more com- 
pact tissue which forms its under shell. These are obviously of advantage to prevent the spread 
of the bone when subjected to the crushing strain. In the sustentaculum tali a bracket-like 
arrangement of fibres is evident, and the under surface of the neck of the astragalus is further 
strengthened by lamelle arranged vertically. 
In the separate bones the investing envelope is thin, though under the articular surfaces 
there is a greater density, due to the accession of lamelle lying parallel to the articular planes. 
The stoutest bony tissue in the astragalus is met with in the region of the under surface of the 
neck, whilst in the os calcis the greatest density occurs along the floor of the sinus tarsi. 
Numerical Variation in the Tarsus.—Increase in the number of the tarsal elements may 
be due to the occurrence of division of either the internal cuneiform or the cuboid bone, or to 
_ the occasional presence of an os trigonum. Cases of separation of the tuberosity of the navicular 
bone have been recorded, and instances of supernumerary ossicles between the internal cuneiform 
and second metatarsal bone have been noted. Stieda mentions the occurrence of a small ossicle 
in connexion with the articular surface on the fore and upper part of the os caleis, and Phitzner 
notes the occurrence of an os sustentaculi. For further information on the variations of the 
skeleton of the foot, see Phitzner (Morphologische Arbeiten, vol. vi. p. 245). 
The reduction in the number of the tarsus is due to the osseous union of adjacent bones. In 
- many cases this is undoubtedly pathological, but cases have been noticed (Leboucq) of fusion of 
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