104. OSTEOLOGY [Parr IT. 
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 MW. 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 exter- 
nal trochlea, half an inch above which it terminates ; between this lime, and the posterior edge of the broad 
external border, is an elongated tract of nearly uniform width, for the origin of the JZ. 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 pos- 
terior 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 the inner margin, les 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 back- 
wards ; it gives origin to the JZ. abductor indicis,' 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 wpper 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 inter- 
condyloid eminence, for the insertion of the external semilunar cartilage. The posterior and external sub- 
acute angle projects outwards, and its upper surface, which slopes backwards, gives attachment to the 
M. peroneus brevis ; anterior to it, isa broad shallow notch ined 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 
! This muscle is not mentioned by Cuvier, Meckel, Owen, &c., although it probably exists in all birds. 
