203 
ever, may be traced to within two inches of the posterior limits 
of the me-sotrochlea. As the ridge subsides a shallow groove 
commences on its inner side (fig. 2, G'), which leads to the inner 
trochlear interspace. 
On the front surface of the upper expansion there is a large 
interosseous depression, with declivous sides (Pl. v., fig. 1, £), 
the upper margin extending to within about an inch and a half 
of the summit of the anterior intercondyloid process. At 
the bottom of this pit are the anterior openings of the inter- 
osseous canals. Immediately below this depression, and encroach- 
ing upon its inferior slope, is a rough vertically striated surface 
for attachment of the tibialis anticus (fig. 1, between H and F). 
Immediately below this rough surface is the upwardly directed 
opening of a nutrient artery (fig. 1, #), to which a slight groove 
leads from below. 
Above the large depression into which the interosseous canal 
opens anteriorly, the surface of the bone is transversely concave, 
and below it, also, the whole of the front surface of the shaft is 
occupied by a wide groove which becomes narrower and shallower 
as it descends ; at a little below the middle of the shaft the 
groove has disappeared, and the front surface is flat transversely, 
below this, again, the same surface becomes more and more trans- 
versely convex with the increasing prominence of the meso-tarsus. 
An ill-defined shallow groove on the front surface of the lower 
third of the shaft, scarcely to be distinguished in the figure, 
leads to the outer intertrochlear interspace. 
On the outer surface of the head there is a prominent, antero- 
posteriorly flattened keel-like process (Pl. v., figs. 1, 3 ZH) 
which commences a little below the outer margin of the articular 
surface, and extends downwards as a crest or ridge for from 2 to 
24 inches. This crest and its ridge-like continuation forms the 
posterior boundary of a shallow groove upon the upper part of the 
outer surface of the ecto-metatarsus. 
The outer side of the hypotarsus, is the large posterior opening 
of the ect-interosseous canal (Fig. 2., J). The opening of its 
fellow on the opposite side is, as has been stated, concealed by a 
bridge of bone. The upper-margin of the bridge is above, and 
the lower below, the level of the eclinterosseous canal. 
Owing to the shape and prominence of the hypotarsus, the upper 
half of the tarso-metatarsus yields a trilateral, indeed 
almost an equilateral, section, the front surface however being reén- 
trant owing to its deep grooving. With the subsidence of the 
hypotarsal ridge, the trilateral section passes into an oval, of 
gradually increasing transverse diameter, as the shaft descends. 
In the middle third the postero-external surface is marked by an 
obscure vertical ridge. 
