1 12 ANATOMY OF THF HORSE. 
Form. Very irregular : large and Hat; broad at the extre¬ 
mities, which turn in different directions ; middle portion con¬ 
tracted. 
Division. Into two surfaces and four borders. 
The External or Dorsal Surface —Divided by the contracted 
portion in the middle into two parts. The anterior part, also 
distinguished as the dorsum ilei, is even and smooth, slightly 
concave, triangular in figure, and faces outwards and back¬ 
wards : it affords attachment to the glutei, maximus et in- 
ternus.—The posterior division presents, outwardly, the ace¬ 
tabulum or cotyloid cavity, hemispherical, nearly three inches 
in diameter, looking outwards and downwards, surrounded 
by a prominent lip of bone which is interrupted below by 
a notch, and having a roughened depression in its middle 
into which is implanted the round ligament confining the 
head of the os femoris within the cavity; above and in front 
of the acetabulum the bone is depressed and roughened in two 
places from the roots of the bifurcated tendon of the rectus fe¬ 
moris ; to the inner side of and below the cavity is the obtura¬ 
tor foramen, a large oval opening, closed by an expansion of 
ligament, through the anterior part of which is a perforation for 
the passage of the obturator vessels and nerves. Behind the 
foramen the surface is broad, even, and smooth, and gives origin 
to the abductor muscles of the thigh ; above the foramen, it is 
smooth and rounded, over which part play the obturator inter- 
nus and gemelli. 
The Internal or Ventral Surface —Divided the same as the 
external. The anterior part faces inwards and forwards, is 
slightly but unevenly convex, has a superficial scabrous depres¬ 
sion posteriorly, shewing the place where the bone rests upon, 
and has strong ligamentous attachment to, the transverse pro¬ 
cesses of the sacrum : the remainder of the surface'* below is 
occupied by the iliacus, the part above being opposed to the 
sacrum.—The posterior part is subdivided by a prominent bor¬ 
der (upon which is a rough mark showing the insertion of the 
psoas parvus) into a small triangular superficial concavity look¬ 
ing forwards which gives origin to the sartorius, and an exten¬ 
sive but irregular one directed upwards ; this last is much con¬ 
sumed by the obturator foramen, posteriorly to which the surface 
is remarkably smooth and slightly excavated for sustaining the 
bladder. 
Anterior Border —Sigmoid in figure, slightly concave and thin 
in the middle; terminating, behind, in a sharp salient angle, the 
posterior iliac spine; before, in a thick quadrangular part which 
gives rise to four eminences : the two larger ones are the antero- 
superior and antero-inferior iliac spines; the two smaller, tubercles 
