edge is faint, the posterior is sharper, and defines centrally the outer limit of the surface for the origin of the
Abductor annularis; below, it becomes faint, and converges towards the anterior.
The internal border extends as a prominent narrow ridge along the inner element in its upper third;
it then turns round towards the posterior aspect, subsiding from the antero-posterior expansion of the centre
of the shaft, the anterior surface being broadly rounded off internally.
The triangular internal surface of the metatarsus has its base above, and extends below to the inner
trochlea; in its upper third, it presents a deep pyramidal excavation for the origin of the M. flexor brevis
pollicis, which also arises from the flat surface, extending beneath the fossa as far as the articular facet for
the posterior metatarsus, situated at the junction of the middle and lower thirds of the shaft. This facet
projects beyond the inner edge, and is covered by a transverse crescentic tract of synovial cartilage.
The external surface is flat, and separated from the inner by the rounded edge of the calcaneal buttress,
which subsides towards the metatarsal articular facet. An intermuscular line descends from the posterior
orifice of the external inter-osseous canal, and, becoming more prominent, sweeps outwards towards the external
trochlea, half an inch above which it terminates; between this line, and the posterior edge of the broad
external border, is an elongated tract of nearly uniform width, for the origin of the M. abductor annularis.
The tendon of this muscle passes over a groove on the outer surface of the peduncle of the external trochlea,
and is bound down by an oblique .annular ligament, attached in front to a small oval tubercle on the outer
edge, and behind to a rough ridge. A second more strongly marked inter-muscular line commences on the
oblique outer aspect nine lines below the preceding, about two lines internal to it, and close to the posterior
border of the subsiding calcaneal ridge, which it crosses as it runs obliquely inwards towards the
articular facet, opposite which it becomes faint; about two lines lower down, it passes into a thick rough
sigmoidal ridge, which terminates four lines above the inner trochlea, and gives attachment to the strong
ligament connecting the metatarsi. An oblique sinus extending to th,e inner margin, lies between this ridge
and the articular facet, and lodges the projecting lower angle of the upper extremity of the posterior
metatarsus. Between this inter-muscular line and that previously mentioned, is a sub-triangular space which
increases in breadth as it descends from the subsidence of the calcaneal buttress; the upper part lies on the
outer surface, extending as high as the ecto-calcaneal process; the lower is deeply concave, and looks backwards;
it gives origin to the M. abductor mdicis,1 the tendon of which is directed towards the inner inter-
trochlear notch, resting in a shallow groove, bounded internally by the outer edge of the elevated posterior
surface of the peduncle of the inner trochlea, and externally by a rough elongated impression parallel to
its inner boundary.
The upper extremity, viewed from above, presents in front the transversely reniform tibial articular
surface, which is broad and rounded internally, narrower externally A semicircular non-articular tract lies
behind in its concavity; the isthmus is raised into the prominent hemispherical intercondyloid tubercle,
separating the inner large and deep subcircular condyloid fossa, from the shallow and smaller external one;
whose anterior edge is bevelled downwards, and internally terminates in a slight pit at the base of the intercondyloid
eminence, for the insertion of the external semilunar cartilage. The posterior and external subacute
angle projects outwards, and its upper surface, which slopes backwards, gives attachment to the
M. peroneus brevis; anterior to it, is a broad shallow notch lined by synovial membrane, immediately beneath
which occurs the oval slightly elevated impression for the insertion of the external lateral ligament. The
anterior angle presents a small transversely elongated tubercle beneath the edge of the condyloid fossa.
The posterior and internal angle presents an oblong subvertical surface directed obliquely outwards and
backwards; its lower and inner angle is tilted upwards, a sharp ridge descending from it on the floor of
1 This muscle is not mentioned by Cuvier, Meckel, Owen, &c., although it probably exists in all birds. •
the fossa for the M. flexor brevis pollicis; this surface gives attachment to the ligament of the great tibio-meta-
tarsai sesamoid fibro-cartilage, which some have regarded, but improperly, as a tarsal element. Anterior to
it is a narrow triangular tract, with the base above; the apex is roughened for the insertion of the
internal lateral ligament, and is prolonged into the ridge-like inner border. Beneath the anterior and
internal angle is the oblique oval prominent tubercle, for the insertion of a ligamentous band, binding down
the tendon of the M. extensor communis digitorum; internally, it is attached to a short rough ridge, along
the inner margin of the anterior concavity, separated from the preceding by a groove leading upwards and
outwards, and impressing the anterior edge of the internal condyloid fossa, as it rises into the intercondyloid
tubercle.
The smooth rounded upper border of the calcaneal ridge {ento-calcaneal process) projects backwards
from the centre of the posterior border, and is raised in the middle into a slight convexity. The posterior
angle of the ridge is expanded and flattened into an irregularly triangular rough plate (calcaneal tuberosity),
with the base above, extending inwards and overhanging the internal muscular fossa, while the apex
protrudes beyond the edge of the buttress; it gives attachment to the tendon of the M. gastrocnemius.
Beneath the upper margin of the ento-calcaneal process, internally, is a curved oblong concavity, separated
by a sharp ridge from the inner muscular fossa: it probably lodges an Haversian gland. The ecto-calcaneal
process is formed by a short, thick plate (one line and two-thirds wide) projecting backwards, nearly
midway between the internal process and the external edge; its free extremity is expanded and broadly
grooved; from the inner lip of the furrow, an osseous bridge passes inwards to join the calcaneal ridge,
converting the space between these processes into a canal {calcaneal carnal), three lines in diameter above,
and seven lines and a half long; below, it contracts slightly, and its inferior orifice is prolonged into a
short triangular groove on the calcaneal ridge. This canal transmits the tendon of the M. flexor perforans
digitorum. The deep groove ^posterior to it, between the calcaneal process and the outer lip of the
ecto-calcaneal furrow, is closed in the recent state by a fibro-cartilaginous bridge; it gives passage to the
tendon of the M. flexor mdicis perforatus anteriorly, and to that of the M. flexor indicis perforam etperfo-
ratus posteriorly. The deep channel for the tendon of the Flexor perforans pollicis,furrows the outer surface
of the ecto-calcaneal process, and is overhung by the projecting outer lip of its groove, from which a
fibro-cartilaginous bridge extends, in the recent state, to a short faint ridge on the posterior surface of
the external and outer angle of the proximal extremity; the groove is thus converted into a canal.
External to the ridge just mentioned, is a short and shallow groove lined with synovial cartilage, over which
plays the tendon of the Peroneus medius (Cuv.), as it proceeds to join fas,perforated tendon of the middle toe.
These , grooves diminish in height from within outwards. In the recent state a broad shallow groove
extends from the outer border of the calcaneal tuberosity to the slightly projecting edge of the external
wall of the canal for the tendon of the perforatiug flexor of the hind toe; its floor is formed externally by
the fibro-cartilaginous roof of that canal, centrally by the groove on the ecto-calcaneal process, and internally
by the fibro-cartilaginous roof of the canal transmitting perforated tendons of the inner toe; it is converted
by the attachment of the fascia to its margins into a canal, which transmits most anteriorly the
tendon of the Flexor medii perforatus, and posteriorly, that of the Flexor medii perforans et perforatus, and
of the Flexor mnularis perforatus, the former being internal, and the latter external.
Viewed from below, the inferior extremity presents the trochleas arranged transversely, so as to form a
small segment of a large circle; seen from before, the inferior surfaces of the two inner trochleas lie in the
same curve, while that of the outer is elevated four lines above the external margin of the middle one, and
is nearly on the same transverse plane as the internal inter-trochlear notch. The elevation of the outer
trochlea, and the abbreviation of the corresponding toe, renders the foot more adapted for progression. The