Tarsal ligaments of the spectacled bear, Tremarctos ornatus

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NATURAL HISTORY.
SURVEY

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* FIELDIANA . ZOOLOGY

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Published by
CHICAGO NATURAL HISTORY MUSEUM

Volume 39 April 21, 1958 No. 13

TARSAL LIGAMENTS OF THE
SPECTACLED BEAR

Tremarctos ornatus

D. Dwight Davis

Curator, Division of Vertebrate Anatomy

Among the Carnivora the tarsal ligaments have been described
for the domestic dog (Baum and Zietzschmann, 1936) and the do-
mestic cat (Reighard and Jennings, 1901). In both these forms the
limbs are greatly modified for cursorial locomotion; the tarsus and
metatarsus are elongated and immobilized, and the tarsal ligaments
are reduced to little more than collateral ligaments. The structure
and mechanics of the ankle and foot differ greatly from those of the
generalized ferungulates, such as the Paleocene Claenodon, that were
supposedly ancestral to the modern Carnivora.

The bears, in contrast, are plantigrade walkers and the bones of
the tarsus do not differ significantly from those of Claenodon. The
foot of the bears closely approximates, in fact, the primitive condition
from which were derived the various specialized conditions found
among modern carnivores and ungulates. It is impossible to under-
stand the mechanics of the tarsus without a knowledge of the tarsal
ligaments, and these ligaments in the bears are therefore of consid-
erable interest. It is remarkable that they have never been examined.

This description is based on the South American spectacled bear,
Tremarctos ornatus. The animal dissected was an adult female
that died in the Zoo in Brookfield, Illinois, and was embalmed for
anatomical study. The tarsal bones of Tremarctos are similar to
those of other bears except that they are somewhat less broadened;
in this respect they approach the primitive condition even more
closely than does Ursus. An X-ray of the specimen before dissection,
and the articulated bones of the tarsus and metatarsus of a second
individual of the same species, along with the tarsal bones of other
species of bears, of other carnivores, and of the fossil Claenodon

Library of Congress Catalog Card Number: 58-9928
No. 838 91

NATURAL
, HISTORY SURVEY

92 FIELDIANA: ZOOLOGY, VOLUME 39

corrugatus, were referred to constantly during the dissection. Com-
parisons were made with the tarsal ligaments of man as described
and figured in standard textbooks and atlases. The nomenclature
of human anatomy has been used wherever possible except that the
term astragalus, customarily used in comparative anatomy, is used
in preference to talus. To avoid unnecessary confusion in the names
of ligaments, talus has been retained as a combining form.

The drawings illustrating this paper were made by Miss Phyllis
Wade, Assistant in the Division of Anatomy, directly from the dis-
sections. Thanks are due the American Museum of Natural History
for the loan of a specimen of Claenodon corrugatus (AM no. 16543).

THE TALOCRURAL ARTICULATION

Only five ligaments are involved in this joint, compared with the
seven present in man. The anterior calcaneotibial and tibionavicular
ligaments are not represented in the bear; in man these are both on
the medial side of the tarsus. The remaining five ligaments are
essentially as in man.

The posterior talofibular ligament is a strong band, circular in cross
section, running diagonally downward and mesad from the posterior
border of the lateral malleolus to the posterior surface of the astrag-
alus just above the groove for the flexor hallucis longus tendon.

The anterior talofibular ligament (fig. 12) is a slender band arising
from the antero-inferior border of the malleolus of the fibula, deep
to the part of the anterior lateral malleolar ligament that passes to
the talofibular meniscus (see below). The ligament runs diagonally
distally and medially across the astragalus, attaching to the superior
surface of the neck of the astragalus immediately behind the talocu-
neiform articulation. A few of its fibers continue distad beyond this
attachment, to attach to the superior surface of the second cuneiform
together with the dorsal talonavicular ligament.

The calcaneofibular ligament (fig. 12) is a strong band of fibers
extending from the calcaneus above and behind the posterior end
of the trochlear process to a well-marked area on the distal end of
the fibula immediately anterior to the malleolar groove. The fibers
along the inferior border of the ligament pass onto the talofibular
meniscus.

The deltoid ligament differs considerably from that of man. Only
two of the four elements are represented in the bear; the longer pair
of ligaments (the calcaneotibial and tibionavicular) are entirely want-

faArf/u*'*- du"

DAVIS: TARSAL LIGAMENTS OF SPECTACLED BEAR

93

Sulcus motaok f*.

malleoli lot ont
Meniscus tolofibulore
Lig tolofibulore oni

Lig lalanaviculoricuneiforme dors
Lig noviculoricuneiforme dors

Lig coiconeofibulore
Sinus tors

Lig totocolconeum lot.

Lig calconeonoviculare

Fig. 12. Ligaments of right tarsus of Tremarctos ornatus, medial view.

ing. The shorter anterior and posterior talotibial ligaments, however,
are present and are very similar to the corresponding structures in man.

The posterior talotibial ligament (fig. 13) is a band of fibers arising
from the distal surface of the medial malleolus of the tibia, immedi-
ately anterior to the malleolar groove. The fibers pass downward
and slightly backward, to insert into the medial surface of the body
of the astragalus.

The anterior talotibial ligament (fig. 13) arises from the anterior
margin of the medial malleolus, directly laterad of the origin of the
posterior talotibial ligament, and passes forward and downward to
the dorsal surface of the neck of the astragalus, below the anterior
part of the medial malleolar surface.

A well-developed interarticular meniscus, the talofibular meniscus
(new name), is situated between the lateral malleolus of the fibula
and the lateral malleolar surface of the astragalus (fig. 12). The
meniscus is a narrow crescent-shaped structure, 14 mm. long by
3 mm. wide. It is wedge-shaped in cross section, the thick outer
edge measuring about 1 mm. The lateral end of the cartilage is held
in place by a tract of fibers that emerges from beneath the calcaneo-
fibular ligament and apparently represents a differentiation from that
ligament; the medial end is held by a tract of fibers lying superficial
to the anterior lateral malleolar ligament and apparently representing
a differentiation from that ligament.

94

FIELDIANA: ZOOLOGY, VOLUME 39

Lig tolodbiole ant
Lig colcaneonovic dors

Tendo m. e»l. hollucis long
retinaculum

Tendo m. tibialis ant.
Lig. tarsometatarseum plont

Os Sesomoideum tibiale

Lig tatocolcan. inteross.
-Sustentaculum calc.
■Lig colcaneonovic. plant.
Tendo m. tibialis post.

Fig. 13. Ligaments of right tarsus of Tremarctos ornatus, laterial view.

The presence of this interarticular meniscus implies lateral thrust
of the astragalus against the fibula. Such thrust would be developed
if the foot were employed in a position of strong inversion, as it is
in climbing.

INTERTARSAL ARTICULATIONS

TALOCALCANEAL LIGAMENTS

The talocalcaneal joint is the key to all tarsal movements except
those in the sagittal plane (extension and flexion). The astragalus
and calcaneus are bound together by five short ligaments, as in man,
and their arrangement and relations are similar to those in man. The
functioning of these ligaments may be compared roughly with the
cruciate ligaments of the knee. In the bear, however, the talocal-
caneal group is dominated by two of these bands, the lateral and
medial talocalcaneal ligaments, which greatly exceed the others in
bulk. The medial in particular differs from the corresponding lig-
ament in man.

The posterior talocalcaneal ligament (fig. 13) is a broad flat band
forming the lateral wall of the groove for the flexor hallucis longus
tendon. The short fibers attach at one end to the astragalus along
the lateral edge of the groove, run backward and slightly downward,
and attach to the medial surface of the neck of the calcaneus a short

DAVIS: TARSAL LIGAMENTS OF SPECTACLED BEAR

95

OS noviculore

Lig colconeonovic
dors

Lig. toiocolc med

Lig. colconeonovic
plant

Smus tarsi

Faoes ortic med
colconei

Os cuboideum

Lig calcaneocuboid
dors

Lig colconeonovic
g toiocolc lot

Y / Focies ortic lot
/^> colconei

Lig toiocolc. mteross
Lig. toiocolc ant

Fig. 14. Ligaments of right tarsus of Tremarctos ornatus, dorsal view, after
removal of astragalus.

distance behind the lateral facet; a slight scar on the bone marks the
site of attachment.

The anterior talocalcaneal ligament (fig. 14) is situated, as in man,
in the sinus tarsi posterior to the interosseous ligament. It attaches
at one end to the calcaneus immediately anterior to the lateral facet,
and at the other to the astragalus along the anteromedial border of
the lateral facet.

The medial talocalcaneal ligament (figs. 14, 15) is a very heavy
rope-like tract of fibers. It is attached at one end to the inferior
surface of the astragalus, in the fovea-like depression between the
navicular articular surface and the medial facet, and at the other end
to the medial surface of the calcaneus immediately in front of the
sustentaculum. Thus the fiber direction is diagonally downward
and outward.

The deep fovea-like depression on the astragalus, reminiscent of
the fovea on the head of the femur, is wanting in man and other
primates. In man its site is occupied by the articular surface for the
plantar calcaneonavicular ligament (this articulation is wanting in
the bear), and in man the medial ligament is shifted back to the
posterior process at the rear of the astragalus. The fovea is present
in other carnivores, in the creodont Claenodon, and at least in Eri-

96 FIELDIANA: ZOOLOGY, VOLUME 39

naceus among the insectivores. Thus it appears that the attachment
site in man (and presumably in other primates) is a secondary one,
as the ligament has been displaced from its original site by the
development of an articulation with the plantar calcaneonavicular
ligament.

The lateral talocalcaneal ligament (figs. 14, 15) is likewise a very
heavy tract of short fibers. It attaches at one end to the lateral

Lig talocalc. lot

Fig. 15. Anterior view of right astragalus and calcaneus of Tremarctos
ornatus, showing relations of lateral and medial talocalcaneal ligaments.

surface of the neck of the astragalus, and at the other to the
superior surface of the calcaneus, in front of the lateral facet; a con-
spicuous scar marks the calcaneal attachment site. Fiber direction,
as for the medial ligament, is diagonally downward and outward.

The lateral and medial ligaments embrace the long axis of the
medial articular facets between them, thus severely restricting an-
teroposterior gliding movements in this part of the joint. They
actually embrace a shifting vertical axis for the lower tarsal joint,
restricting movements between the astragalus and calcaneus almost
entirely to rotation around this axis. The result is that major excur-
sion in the lower tarsal joint is between the two lateral facets, which
are longer than the medial facets to accommodate this movement.
Because of the arching of the lateral facets, rotatory movements in
the lower tarsal joint are translated into eversion-inversion move-
ments of the foot.

The interosseous ligament (fig. 14) is similar to the corresponding
ligament of man. It lies in the sinus tarsi and consists of a long tract
of short fibers extending between the astragalar and calcaneal
grooves.

DAVIS: TARSAL LIGAMENTS OF SPECTACLED BEAR 97

DORSAL TARSAL LIGAMENTS

A dorsal talonaviculocuneiform ligament (fig. 12) represents the
dorsal talonavicular ligament of human anatomy. In the bear it is
a flat band running diagonally across the dorsal surface of the foot,
from the anterior surface of the body of the astragalus lateral to the
neck, to the dorsal surface of the second cuneiform. The ligament
is attached to the superior surface of the navicular as it passes over
that bone, but its main attachment is to the cuneiform.

The bifurcate ligament of man is represented in the bear only by
the superior (calcaneonavicular) part; the inferior (calcaneocuboid)
part is wanting. The pars calcaneocuboidea is often wanting in man
(Braus, 1929). The calcaneonavicular ligament (fig. 14) is a thin flat
band running across the dorsum of the foot, from a point just above
the distal end of the trochlear process of the calcaneus to the dorso-
lateral corner of the navicular. Its medial end passes deep to the
anterior talonavicular ligament.

The dorsal calcaneocuboid ligament (fig. 14) is a short flat band
extending anterolateral^ from the anteromedial angle of the coracoid
process of the calcaneus to the superior surface of the cuboid near its
posteromedial angle.

The dorsal cuboideonavicular ligament is a short band extending
transversely between the lateral border of the navicular and the
adjacent border of the cuboid on the dorsum of the foot.

The dorsal naviculocuneiform ligaments are represented by three
isolated short bands extending between the anterior border of the
dorsum of the navicular and the superior surfaces of the cuneiforms,
one ligament passing to each of the three bones. The broadest lig-
ament is associated with the third cuneiform.

PLANTAR TARSAL LIGAMENTS

The ligaments on the plantar surface differ considerably from
those in man, not only in detail but also in basic pattern. Fibro-
cartilaginous sesamoid cartilages are more extensively developed than
in man. The navicular fibrocartilage of human anatomy is represented
by a large nodule situated beneath the medial end of the navicular,
the first cuneiform, and the tibial sesamoid. It is associated with the
insertion of the tendon of the posterior tibial muscle and the ventral
fibers of the plantar calcaneonavicular ligament. There is also a
group of metatarsal fibrocartilages beneath the proximal ends of
metatarsals 2-4, which are associated with the attachments of the

98 FIELDIANA: ZOOLOGY, VOLUME 39

long plantar, the medial plantar, and the plantar tarsometatarsal
ligaments.

The long plantar ligament (figs. 12, 16) is the largest tarsal liga-
ment. It is composed of two subequal parts, a lateral part passing
over two joints and a compound medial part that extends over one
joint at a time. The two parts are inseparable proximally, arising
as a unit from the prominent anterior tubercle on the inferior surface
of the calcaneus near the cuboid articulation. The lateral part passes
over the ventral tuberosity of the cuboid without attaching to the
bone, and inserts into the ventral tuberosity at the base of the fifth
metatarsal.

The medial part of the long plantar ligament is interrupted by
the ventral tuberosity of the cuboid, which divides it into proximal
and distal parts. The proximal fibers, arising from the anterior
tubercle of the calcaneus, insert into the proximal surface of the
cuboid tuberosity. The distal fibers arise from the distal border of
the cuboid tuberosity and insert into the metatarsal fibrocartilage
underlying the base of the fourth metatarsal.

The medial plantar ligament (new name) is not represented in man.
It lies parallel to and immediately mesad of the long plantar liga-
ment, separated from the latter by an interval (fig. 16). Like the
long plantar ligament, it is composed of a long and a short element.
These have a common proximal origin from the antero-inferior border
of the calcaneus directly above the groove for the flexor hallucis
longus. The long lateral part passes over the tuberosity of the
navicular without attaching to the bone, and inserts into the meta-
tarsal fibrocartilage beneath the base of metatarsal 3. The short
medial part attaches to the proximal and inferior surfaces of the
tuberosity of the navicular.

The plantar calcaneocuboid ligament (short plantar ligament) is
completely covered by the long plantar ligament. It is a short band
attached to the anterior surface of the calcaneus between the ante-
rior tubercle and the cuboid articular surface, and passes straight
forward to attach to the inferior surface of the cuboid between the
tubercle and the calcaneal articular surface.

The plantar calcaneonavicular ligament (figs. 13, 14, 16) is a broad
thick plate arising from the whole anterior border of the sustentacu-
lum and inserting into the medial part of the inferior surface of the
navicular, the proximal surface of the tibial sesamoid, and the navic-
ular fibrocartilage. Dorsomedially it is blended with the inferior
edge of the dorsal calcaneonavicular ligament. The general fiber

DAVIS: TARSAL LIGAMENTS OF SPECTACLED BEAR

99

direction is anteroposterior. The ligament bridges the medial part
of the notch between the sustentaculum and the navicular; the lateral
part of this notch is filled with fat, connective tissue, and the medial
talocalcaneal ligament (fig. 14). The inner surface is cupped to
receive the head of the astragalus. Thus the ligament would appear
to form a functional part of the socket in which the head of the
astragalus articulates, as it does in man. In the bear there is no
articular surface on the opposing area of the astragalus, however,
and it is therefore evident that the ligament does not help support
the down thrust of the astragalus head as it does in man. A large
nodule of fibrocartilage, the navicular fibrocartilage of human anat-
omy, is embedded in the ligament between the medial ends of the

Lig tok>hb:ole post

Lig colcooeooovic plant
Tendo m tibialis post

Ligg torsometotorseo
plant

Os sesomoideum tibial*
Os cuntiforme I (fibrocortiloge
covering)

Ligg novicukvicuntif
plant

Lig plontore med.

Tuberos. ossn novic
A(\. Lig plontor* long

Fig. 16. Ligaments of right tarsus of Tremarctos ornatus, plantar view.

100 FIELDIANA: ZOOLOGY, VOLUME 39

sustentaculum and the navicular; distally it is embraced between
the tuberosity of the navicular and the tibial sesamoid. The medial
face of the ligament is grooved for the posterior tibial tendon, which
does not pass beneath the ligament and provide additional support
for the astragalar head as it does in man.

The plantar cuboideonavicular ligament is a short transverse band
running from the lateral surface of the navicular tubercle to the
medial surface of the cuboid tubercle. It lies deep to the plantar
calcaneonavicular ligament.

The plantar navicular icuneijorm ligaments (fig. 16) are represented
by two distinct sets of short bands. One extends anteriorly and dor-
sally from the anterior border of the navicular tuberosity to the
inferior surfaces of the second and third cuneiforms. The other
extends medially from the medial border of the navicular tuberosity
to the inferior surface of the first cuneiform near its posterior end.

RELATIONS OF THE TIBIAL SESAMOID

The bears, like most pentadactyl mammals, have an accessory
bone, the so-called tibial sesamoid, on the medial side of the second
row of tarsals (figs. 13, 16). In Tremarctos this bone articulates
about equally with the navicular and the first cuneiform. This is
not a true diarthrosis; there is no articular cavity and the contact
surfaces of the bones are not covered with articular cartilage. In-
stead, the sesamoid is tightly bound to the navicular and first cunei-
form by tough fibrocartilage, which is particularly heavy at the
periphery but is also present over the contact surfaces.

To the tibial sesamoid are attached certain tendons and ligaments
that in man attach elsewhere. It serves primarily as the principal
site of insertion for the tendon of the posterior tibial muscle, which
in man inserts chiefly into the tubercle of the navicular bone.1 In
Tremarctos the tibial sesamoid is held in place by two ligaments, in
addition to the fibrocartilage that binds it directly to the navicular
and first cuneiform : the lateral fibers of the plantar calcaneonavicular
ligament attach to its proximal face, and the medialmost element of
the plantar tarsometatarsal ligament to its distal face. A retinacu-
lum for the tendons of the tibialis anterior and extensor hallucis
longus attaches to its external face. These relations are essentially
the same in other mammals (Carlsson, 1891).

1 Whether or not the sesamoid often found in this tendon in man is homologous
with the tibial sesamoid of quadrupeds seems to me a sterile argument.

DAVIS: TARSAL LIGAMENTS OF SPECTACLED BEAR 101

The functional significance of the tibial sesamoid, and of its
counterpart in the forefoot, the radial sesamoid, is not clear. The
absence of an articular cavity shows that the bone is essentially im-
mobile. Its primary purpose would therefore seem to be to broaden
the tarsus, although the advantage of broadening the tarsal complex
in this peculiar way is not evident.

TARSAL MOVEMENTS

Attempts to localize and verify movements of the tarsal bones by
means of superimposed X-ray photographs are not entirely success-
ful because of the small size of the bones and the complexity of the
movements. Tracings of the photographs (figs. 17, 18) do verify
deductions from the dissection and from manipulation of tarsal
bones. Really accurate analysis of these movements in the bear
would require the use of targets mounted on the ends of long radii
inserted into the bones, a technic developed by Close and Inman
(1953) in their work on the human tarsus.

DISCUSSION

The tarsus of the bear is a relatively unspecialized mammalian
structure, the bones differing little from those of generalized ferungu-
lates of Paleocene age. The only plantigrade mammal whose tarsal
ligaments are known is man, and consequently it is with man that
the ligaments of the bear must be compared. To find an ancestor
common to the Carnivora and the Primates it is necessary to go back
at least 80 million years, into the Cretaceous.

The general pattern of the tarsal ligaments is very similar between
Tremarctos and man. There are several differences, however, and
these are intimately related to differences in the mechanics of the
tarsus. They are:

1. In the talocrural (upper tarsal) joint two ligaments found in
man are absent in the bear: the calcaneotibial and tibionavicular.
In man these both lie on the medial side of the joint, and both pass
over two joints. Thus they strengthen the ankle assembly, but they
do so at the expense of flexibility.

2. In the talocalcaneal (lower tarsal) joint of the bear the power-
ful lateral and medial talocalcaneal ligaments are as important as the
articular surfaces on the bones in determining the nature of joint
movements. By fixing a slightly shifting center of rotation just an-
terior to the medial facets, these ligaments permit rotatory move-

Fig. 17. Tracings from X-ray photos of right tarsus and pes of Tremarctos
ornatus in positions of inversion and eversion. Embalmed specimen; muscles
removed, ligaments intact; tibia clamped in vise and foot manipulated from distal
end. The black lines show the foot in eversion, the red lines in inversion, with the
outlines of the tibia superimposed. In the upper tarsal joint there is very slight
rotation of the astragalus around the long axis of the tibia. In the lower tarsal
joint the maximum permissible rotation of the calcaneus takes place around the
astragalus. Rotation is around a vertical axis passing obliquely downward and
outward through the trochlea of the astragalus, then through the calcaneus ante-
rior to the medial facet. The site of this axis is determined largely by the lateral
and medial talocalcaneal ligaments. In the transverse tarsal joint the navicular-
cuboid unit rotates around a longitudinal axis passing proximo-distally through
the interspace between the two bones. The range of inversion-eversion movements
of the foot results from additive movements of both the lower and transverse
tarsal joints.

102

Fig. 18. Tracings from X-ray photos of right tarsus and pes of Tremarctos
ornatus in positions of extension and flexion. Data as for figure 17. Movement
is exclusively in the upper tarsal joint. Extension is accompanied by a slight
displacement forward and upward of the fibula with relation to the tibia.

103

104 FIELDIANA: ZOOLOGY, VOLUME 39

ments of the astragalar head — movements vital to eversion-inversion
movements of the foot. They do not, however, allow the head to be
displaced significantly from its position vis-a-vis the cuboid articu-
lation on the calcaneus — a feature vital to the integrity of the four-
bone complex involved in the transverse tarsal joint. Thus these
ligaments, as much as the articular facets themselves, are a key to
an understanding of the mechanics of the bear's tarsus. Nothing
comparable is found in the tarsus of man.

3. In man the major plantar complex is concerned primarily with
the structural stability of the longitudinal and transverse arches of the
foot. It is long and narrow, concentrated along the axis of the long
plantar ligament; the axis passes through the third metatarsal (which
is very nearly the plane of balance of the human foot). In the bear
there is no longitudinal arch and no functional transverse arch. The
plantar complex is short and broad, including a ligament (the medial
plantar) not found in man, and with its axis more laterad than in
man, passing through the fifth metatarsal.

In man the head of the astragalus is supported from below by the
plantar calcaneonavicular ligament, and bears a special articular area
for this articulation. Failure of this ligament results in "a kind of
flat-footedness, in which the talus sinks down to the sole surface"
(Braus). The astragalar attachment of the medial talocalcaneal lig-
ament is on the posterior part of the bone. In the bear the site of
the human calcaneonavicular articulation on the astragalar head is
occupied by the attachment of the medial talocalcaneal ligament,
and the astragalar head is not supported from below by the plantar
calcaneonavicular ligament; the ligamentary part of the "acetabu-
lum" consists mostly of a medial wall.

In man the posterior tibial and long peroneal tendons together
form a stirrup that supports both arches. In the bear neither of
these tendons reaches the plantar surface, and consequently they
provide no such support.

4. The tibial sesamoid is a primitive mammalian structure of
uncertain functional significance. Its absence in man represents a
departure from the primitive condition.

In general the tarsus of the bear is a far more flexible structure
than its human counterpart. Even the upper tarsal joint (between
tibia and astragalus) appears to be less rigidly restricted to hinge
movement than in man. Range of movement in the lower tarsal
joint (between astragalus and calcaneus) is notably greater than in
man, and this is likewise true of the transverse tarsal joint. As a

DAVIS: TARSAL LIGAMENTS OF SPECTACLED BEAR 105

supporting structure, on the other hand, it is more poorly designed
for distributing stresses and strains, and thus for enhancing the
stability of the limb as a whole.

SUMMARY

1. The tarsal ligaments of the spectacled bear are basically similar
to those of man.

2. Three ligaments found in man — the calcaneotibial, the tibio-
navicular, and the calcaneocuboid — are wanting in the bear.

3. One ligament found in the bear — the medial plantar — is want-
ing in man. This ligament is associated with a broad short plantar
complex that contrasts with the narrow long complex of man.

4. In the bear an interarticular meniscus is situated in the talo-
fibular joint.

5. In the bear the medial and lateral talocalcaneal ligaments are
extremely heavy, embrace the head of the astragalus, and are the key
to movements in the lower tarsal joint. In man the medial ligament
has been displaced backward from its original attachment site on the
head of the astragalus.

6. The tarsus of the bear is a more flexible but less stable structure
than is the tarsus of man.

REFERENCES

Baum, Hermann, and Zietzschmann, Otto

1936. Handbuch der Anatomie des Hundes. I. Skelett- und Muskelsystems.
Berlin: Parey; viii, 242 pp., 180 figs.

Braus, Hermann
1929. Anatomie des Menschen. Ed. 2. 1, Bewegungsapparat. Berlin:
Springer; xi, 822 pp., 387 figs.

Carlsson, Albertina

1891. Untersuchungen iiber die weichen Teile der s.g. iiberzahligen Strahlen
an Hand und Fuss. Bihang K. Svenska Vet.-Akad. Handl., 16, Aid. 4, no. 8,
pp. 1-40, 4 pis.

Close, J. R., and Inman, V. T.

1953. The action of the subtalar joint. Prosthetic Devices Res. Proj., Univ.
Calif., ser. 2, Issue 24, 7 pp., 12 figs.

Morton, D. J.
1935. The human foot; its evolution, physiology and functional disorder.
New York: Columbia Univ. Press; xiii, 244 pp., 100 figs.

Reighard, Jacob, and Jennings, H. S.

1901. Anatomy of the cat. New York: Holt; xx, 498 pp., 173 figs.