S5 S A N A 1 
figure of the heart with its point downward?. The ante¬ 
rior convex fiirfaee of the rotula is pierced by a great 
number of holes, into which fibres of the ftrong ligament, 
that is fpread over it, enter. Behind, its furface is fmooth, 
covered with cartilage, and divided by a middle convex 
ridge into two cavities, of which the external is large ft ; 
and both are exafUy adapted to the pulley of the os fe- 
moris, on which they are placed in the mod ordinary 
unftraining poftures of the leg: but, when the leg is 
much bent, the rotula defcends far down on the con¬ 
dyle' ; and, when the leg is fully extended, the rotula riles 
higher in its upper part than the pulley of thigh-bone. 
The plain fmooth furface is furrounded by a rough promi¬ 
nent edge, to which the capfular ligament adheres. Be¬ 
low, the point of the bone is fcabrous, where the ftrong 
tendinous ligament from the tubercle of the tibia is fixed. 
The upper horizontal part of this bone is flatted and un¬ 
equal, where the tendons of the extenfors of the leg are 
inferted. The fubftance of the rotula is cellular, with 
very thin external firm plates; but then thefe cells are fo 
fmali, and fuch a quantity of bone is employed in their 
formation, that fcarcely any bonb of its bulk is fo ftrong. 
Befides, it is covered all over with a thick ligament, to 
connect its fubftance, and is moveable to either fide; 
therefore it is efficiently ftrong to relift the ordinary ac¬ 
tions of the large mufcles, or any common external force. 
On account of the very large furface of the bones form¬ 
ing the joint of the knee, and the many ftrong ligaments 
connecting them, luxations feldom happen here. But 
thefe very ligaments, the aponeurofis palling over this joint, 
and the quantity of fat and mucilaginous glands neceflary 
for lubri ating it, make it more fubjedl to white fwellings, 
drop ties, and l'uch other diforders, than any other joint of 
the body. 
The foot is divided, as well as the hand, into three parts, 
viz. tar [is, metatarfis , and toes. The tarjus confifts of fe- 
ven fpongy bones, viz. the aftragalus, os calcis, navicu- 
lare, cuboides, cuneiforme extermim, cuneiforme medium, 
and cuneiforme internum. The upper part of the ajlra- 
n-alus is formed into a large fmooth head, which is (lightly 
hollowed in the middle ; and therefore refembles a fuper- 
ficial pulley, by which it is fitted to the lower end of the 
tibia. The internal fide of this head is flat and fmpoth, 
to play on the internal malleolus. The external fide has 
alio luch a furface, but larger, for its articulation with 
the external malleolus. The lower furface of the aftra- 
oalus is divided by an irregular deep rough folia ; which 
at its internal end is narrow, but gradually widens as it 
liretches obliquely outwards and forwards. The fmooth 
furface, covered with cartilage, behind this folia, is large, 
oblong, extended in the fame oblique fituation with the 
folia, and concave for its conjunction with the os calcis. 
The back-part of the edge of this cavity is produced in¬ 
to two (harp-pointed rough procefles ; between which is a 
deprefiion that is made by the tendon of the flexor pol- 
licis longus. The lower furface of it before the folia 
is convex, and compofed of three diftinCt fmooth planes. 
The long one behind, and the exterior or fhorteft, are ar¬ 
ticulated with the heel-bone; while the internal one, 
which is the moft convex of the three, refts and moves 
upon a cartilaginous ligament, that is continued from the 
calcaneum to the os Icaphoides. Without which liga¬ 
ment the aftragalus could not be fuftained; but would be 
pro lied out of its place by the great weight it fupports, 
and the other bones of the tarfus would be fcparated. The 
fore-part of this bone is formed into a convex oblong fmooth 
head, called by fome its procejs, which is received by the 
os naviculare. Round the root of this head, efpecially on 
the upper furface, a rough folia may be remarked. The 
aftragalus is articulated above to the tibia and fibula, which 
together form one cavity. Though in this articulation the 
tones.have prominences and cavities, fo fmali as might al¬ 
low motions in all directions, yet the flexion and extenlion 
are the lftoff confiderable, the other motions being con¬ 
fined by the malleoli, and, by-the ftrong ligaments which 
O M Y. 
go out from the points of tliefe procefles to the aftragalus 
and os calcis. 
The os calcis is the largeft bone of the feven. Behind, 
it is formed into a large knob, commonly called the heel: 
the furface of which is rough behind, where the tendon 
Achillis is inferted ; and above that part it is hollow and 
fpongy. Farther forwards, on the upper furface' of the 
calcaneum, there is an irregular oblong fmooth convexity, 
adapted to the concavity at the back-part of the aftraga¬ 
lus : and beyond this a narrow folia is 1'een, which divides 
it from two fmali concave fmooth furfaces, that are joined 
to the fore-part of the aftragalus. Behind the pofterior of 
thefe fmooth furfaces, which is the largeft, a fmali finuo- 
lity is made by the tendon of the flexor digitorum longus; 
at the fore-part of which a fmali rough protuberance ap¬ 
pears, that gives rife to the mufculus extenfor digitorum 
brevis. The external fide of this bone is fiat, with afu- 
perficial folia running horizontally, in which the tendon 
of the mufculus peronaeus longus is lodged. The inter¬ 
nal fide of the heel-bone is hollowed, for lodging the ori¬ 
gin of the mafl'a cornea Jac. Sylvii, and for the lafe paf- 
fage of tendons, nerves, and arteries. Under the lide 
of the internal fmooth cavity, a particular groove is made 
by the tendon of trie flexor pollicis longus ; and, from the 
thin protuberance of this internal fide'the cartilaginous li¬ 
gament that fupports the aftragalus goes out to trie os na¬ 
viculare; on which ligament, and on the edge of this 
bone to which it is fixed, the groove is formed for the ten¬ 
don of the flexor digitorum profundus. The lower fur¬ 
face of this bone is prefled flat at the back-part, by the 
weight of our bodies; and, immediately before this plane, 
there are two tubercles, from the internal of which tire 
mufculus abdulfpr pollicis, flexor digitorum fublimis, as 
alfo part of the aponeurefis plantaris, and of the abductor 
minimi digiti, have their origin ; and the other part of the 
abdudtor minimi digiti and aponeurofis plantaris riles from 
the external. Befgre thefe protuberances this bone is con¬ 
cave, for lodging the flexor mufcles; and at its fore-part 
we may obferve a rough deprelfion, from which, and a tu¬ 
bercle behind it, tire ligament goes out that prevents this 
bone from being feparated from the os cuboides. The 
fore-part of the os calcis is formed into an oblong pulley¬ 
like fmooth furface, which is circular at its upper exter¬ 
nal end, but is pointed below. The fmooth furface is fit¬ 
ted to the os cuboides. 
Os naviculare, is fomewhat circular. It is formed into 
an oblong concavity behind, for receiving the anterior 
head of the aftragalus. On the upper furface there is a 
rough folia. Below, the os naviculare is very unequal and 
rough; but hollow for the fafety of the mufcles. On its 
inftde a large knob riles out, from which the abduftor pol¬ 
licis takes in part its origin, the tendon of the tibialis pof- 
ticus is inferted into it, and to it two remarkable ligaments 
are fixed; the ill ft is the ftrong one, which fupports the 
aftragalus; the fecond is ftretched from tiiis bone obliquely 
acrols the foot, to the metatarfal bones of the middle toe, 
and of the toe next to the little one. On the outlide of 
the os naviculare there is a femicircular fmooth furface, 
where it is joined to the os cuboides. The fore-part of 
this bone is all covered with cartilage, and is divided into 
three fmooth planes, fitted to the three ofla cuneiformia. 
The os naviculare and aftragalus are joined as a ball and 
focket; and the naviculare moves in all airedlions in turn¬ 
ing the toes inwards, or in railing or deprefling either fide 
of the foot, though the motions are greatly reftrained by 
the ligaments which conned! this to the other bones of the 
tarfus. A weaknefs of thefe ligaments caufes fometimes 
an unnatural turn of the fore part of the foot inwards. 
Os cuboides is a very irregular cube. Behind, it is form¬ 
ed into an oblong unequal concavity, adapted to the fore¬ 
part of the os calcis. On its internal fide, there is a fmali 
femicircular fmooth cavity, to join the os naviculare. Im¬ 
mediately before which, an oblong fmooth plane is made 
by the os cuneiforme extermim. Below triis the bone is 
hollow and rough. On the internal fide of the lower fur- 
3 face 
