directed vertically; its anterior surface is slightly convex
transversely. This surface is covered by a number of tendons, which
slightly alter its appearance; so that it is the principal base of this
part of the fore-limb. Its posterior surface is flattened. The superior
extremity of this metacarpal presents plane surfaces, variously
inclined, with which the bones of the inferior row of the carpus
articulate. On the anterior surface, and a little to the inner side, is
found a tuberosity, which is destined for the insertion of _the anterior
extensor of the metacarpus_, the homologue of the radial muscles. The
inferior extremity is formed by two condyles, an internal and an
external; between which is found a median crest.
This extremity, the superior extremity of the first phalanx, which
articulates with it, together with two sesamoid bones--the great
sesamoids--which are situated on its posterior surface, collectively
form the region which from its rounded outlines is called the _ball_.
With regard to the rudimentary metacarpals, external and internal, to
which some authors give the name of _fibulae_, they are applied to the
sides of the posterior surface of the principal metacarpal. They are
elongated bones, of which the superior extremity, which is a little
thickened, is called the _head_; the lateral bones of the second row of
the carpus partly rest on the heads of these. They become more slender
as they descend, and terminate opposite the inferior fourth of the
principal metacarpal. Each ends in a slight swelling, to which the name
_button_ has been given. The internal one is the better developed.
The rudimentary metacarpals are vestiges of atrophied digits, as will be
explained further on.
The single finger of the horse consists of three phalanges. The first
phalanx, which is directed obliquely downwards and forwards, corresponds
to the constricted region situated below the 'ball,' and known as the
_pastern_. It is flattened from before backwards; its anterior surface
is convex transversely, while the posterior surface is plane. Its
superior extremity is moulded on the inferior extremity of the principal
metacarpal, and its inferior extremity, which is smaller, presents a
trochlea with which the second phalanx articulates. This is also
directed downwards and forwards, and is shorter. It corresponds to the
region which, situated between the pastern and the hoof, is known as the
_cornet_.
The third phalanx, situated entirely within the hoof, has the same
direction as the first and second. It is large and broad, and presents
three surfaces separated by well-marked angular borders (see Fig. 96).
The anterior surface is oblique downwards and forwards; it is convex
transversely. The inferior surface is slightly hollowed, and is in
relation with the sole, or plantar surface of the hoof.
The superior surface, which is articular, is divided by a median ridge
into two lateral cavities, which correspond to the trochlea on the
inferior surface of the lower extremity of the second phalanx. The
inferior border corresponds in shape with the hoof. The superior border
presents in its median part a projection, _the pyramidal eminence_,
which prolongs at this level the anterior surface of the bone. Finally,
the posterior border, which is concave, is in contact with a sesamoid
bone, _the lesser sesamoid_, which increases the superior articular
surface behind, and is also in contact with the second phalanx.
As we have just seen, the horse possesses but one digit. In the
ancestors of the animal--that is, in the prehistoric species which
are now extinct (_orohippus_, _miohippus_, _protohippus_, or
_hipparion_)--the number of digits was larger; this fact conclusively
proves that the rudimentary metacarpals of the existing horse are
vestiges of digits which have disappeared through want of use. In the
first of those ancestors--orohippus--there were four digits; all save
the first, the thumb, being then developed. In the others of the series
there existed but three digits. It must, however, be noted that in those
animals it is always the digit which corresponds to the middle finger of
the pentedactyl hand that is longest. In other less ancient species the
lateral fingers are reduced to the condition of mere splints of bone. It
follows from what has been said that the digit which persists in the
equine species should be considered as the third finger, and that the
rudimentary metacarpals represent lateral digits considerably
atrophied.
This disappearance of the lateral digits cannot excite surprise when we
consider the functions of the organs. Becoming useless, they must
undergo gradual atrophy from want of use.
There undoubtedly is, in this former existence of supplementary digits
in the horse, something analogous to what we still find in the pig;
where the two principal digits are accompanied by two shorter ones,
which very probably, from their infrequent use, are destined to
disappear in a more or less distant future.
Proportions of the Arm, the Forearm, and the Metacarpus
As a supplement to the study of the anterior limbs which we have just
finished, it appears necessary to give some indications of the relative
proportions of certain of the segments which form these limbs in the
plantigrades, the digitigrades, and the ungulates.
First, we would remark that, in following this order of classification,
the scapula becomes less and less narrow, and assumes a form more and
more elongated. In order to convince ourselves of this, it will be
sufficient to study the bone first in man, then in the bear, the cat,
dog, ox, and finally in the horse.
As to the proportions of length, which are those we should chiefly
study, we shall commence with the comparison of the forearm and
arm--that is to say, the radius and the humerus. The radius is found to
be longer in proportion to the humerus, as the number of digits is
smaller, and the hand loses more and more the functions of an organ of
prehension. In man, the radius is shorter than the humerus; in the
horse, on the contrary, it is longer.
To give an idea of this proportion, we shall employ what is known as the
antibrachial index. This index gives the relation which exists between
the length of the forearm and that of the humerus; the length of this
latter, whatever may be the actual measurement, is represented by a
fixed figure, the number 100. A very simple arithmetical operation
gives the proportion--
forearm x 100
-------------, the quotient obtained furnishes the index.
humerus
The index is less than 100 if the forearm is shorter than the bone of
the arm. The index is more than 100 if, on the contrary, the forearm is
longer.
In man, the radius is shorter than the humerus; indeed, in adult
individuals of the white race the average index is 74.
In the bear, the length of the radius approaches closely to that of the
humerus; the index is about 90. In the skeleton of a bear in the
anatomical museum of the Ecole des Beaux-Arts, the humerus is 33
centimetres in length, and the radius 30 centimetres.
In the cat, the radius is very little shorter than the humerus. In the
dog they are equal. The antibrachial index of the latter is,
accordingly, 100.
In the horse, the radius is longer than the humerus; the index is
therefore above 100. Thus, in the skeleton of the horse which we have in
the museum of the Ecole des Beaux-Arts, the index is 113--length of
humerus, 29 centimetres; length of radius, 33 centimetres. In other
skeletons which we have measured we found: in one, 108--humerus, 34
centimetres; radius, 37 centimetres; in another, 116--humerus, 25
centimetres; radius, 29 centimetres.
The metacarpal bone undergoes, relatively to the humerus, a proportional
elongation, analogous to that of the forearm.
In man, the length of the metacarpus is contained about 5-1/2 times in
that of the humerus; in the bear, it is contained 4 times; in the dog,
2-1/2 times; in the horse, 1-1/3 times only.
It is well known that the proportions vary according to race, and that
what we have here given are but the general indications.
The Articulations of the Anterior Limbs
The knowledge of human arthrology which we presume the reader to have
previously acquired makes it unnecessary for us to enter into numerous
details regarding the configuration of the articular osseous surfaces
and the disposition of the fibrous bands that retain them in position.
Accordingly, in the description which follows, and also in that of the
articulations of the posterior limbs, we shall occupy ourselves but very
briefly with the details above referred to, so as to devote ourselves
especially to the indication of the movements--that is to say, of that
which, while easily comprehended on recollection of former studies,
presents the greatest interest from the artistic standpoint in these
studies in comparative anatomy.
=The Scapulo-Humeral Articulation.=--The head of the humerus and the
glenoid cavity of the scapula being in contact, the two bones are bound
together by a rather loose articular capsule, which is strengthened by
the muscles of this region which fulfil the function of active
ligaments.
This articulation, so movable in every direction in the human species,
is not so much so in quadrupeds; the arm in the latter, as also the
shoulder, being kept in contact with the lateral region of the thorax by
the numerous muscles which surround it.
Of the movements performed by the humerus, flexion and extension are the
most extensive; those of abduction and adduction are much less so.
It is necessary, before proceeding further, to determine what the two
principal movements which we have just mentioned really are, viz.,
flexion and extension.
We know that in man the displacements of the humerus which take place in
the antero-posterior direction are known as movement or projection
forwards, and movement or projection backwards, respectively. We do not
say that the humerus is flexed or extended, because, in reality, on
account of the position which the skeleton of the shoulder occupies, it
is not able to flex or place itself on the line of prolongation of the
scapula with which it articulates.
In quadrupeds it is not so. The humerus and the scapula are contained in
almost the same vertical plane; and the bone of the arm can take, in
relation to the latter, the positions characteristic of flexion and
extension--that is, of approach to the scapula and removal from it.
What makes the meanings of these terms a little confusing is that, in
human anatomy, some authors consider the backward movement of the
humerus as extension, and the forward movement as flexion; in order to
be able to compare these movements to those that the femur executes in
relation to the pelvis.
Now, in our opinion, the indication of this correspondence is not
absolutely necessary; since it ceases to be exact if we wished, from the
point of view of the direction given to other segments of the skeleton,
to establish the same relation between the elbow and the articulation of
the knee.
It is therefore indispensable, when discussing quadrupeds, to
discontinue these terms, in order the more readily to recognise that: in
flexion the inferior extremity of the humerus is directed backwards; in
extension, on the contrary, it is directed forwards. In the first case
the humerus approaches the scapula; in the second, on the contrary, it
moves away from it.
These movements, which take place during walking, are executed in the
following manner: When one of the anterior limbs is at the end of that
stage of progression which is called support (see p. 289, Displacements
of the Limbs)--that is to say, during the time that the foot remains in
contact with the ground, whilst the trunk is moving forward--the
direction of this limb becomes more and more oblique downwards and
backwards. At a certain moment the limb is raised from the ground, to be
carried forwards, in order to be again pressed on the ground, and
recommence a new resting stage. In these different phases the humerus is
flexed. But at the moment that the limb, when carried forwards, is about
to resume its contact with the ground it becomes directed obliquely
downwards and forwards; then the humerus is in the position of
extension.
During these movements of the humerus, there exists an essential
factor--that is, the scapular balance. (It is the same as what occurs in
man when he balances his arm in the antero-posterior plane.) When the
humerus is flexed, the scapula moves in such a way that the superior
portion projects forwards; when it is extended, the scapula, on the
other hand, is inclined more backwards. But it is necessary to add that,
during these displacements, the scapulo-humeral angle varies; it tends
to close during the flexion of the humerus, and becomes more open during
extension.
The movements and the relations of the humerus and the scapula are
clearly represented in Figs. 41 and 42, reproduced from the
chromophotographic studies of Professor Marey--studies relative to the
analyses of the movements of the horse.[13] They show clearly the
movements of flexion and extension of the humerus, also the balancing of
the scapula which accompanies the movements.
[13] E. J. Marey, 'Analyses of the Movements of the Horse by the
Chromophotograph' (_La Nature_, June 11, 1898).
[Illustration: FIG. 41.--FLEXION OF THE HUMERUS: RIGHT ANTERIOR LIMB OF
THE HORSE, EXTERNAL SURFACE (AFTER A CHROMOPHOTOGRAPHIC STUDY BY
PROFESSOR MAREY).]
[Illustration: FIG. 42.--EXTENSION OF THE HUMERUS: RIGHT ANTERIOR LIMB
OF THE HORSE, EXTERNAL SURFACE (AFTER A CHROMOPHOTOGRAPHIC STUDY BY
PROFESSOR MAREY).]
=The Articulation of the Elbow, or the Humero-ulnar Articulation.=--In
this articulation, which is constructed in the form of a true hinge, the
movements of flexion and extension alone are possible. In flexion, the
forearm, directed forwards, is folded on the arm, with which, in certain
circumstances, it comes in contact. For example, in a horse of mettle
which leaps over an elevated obstacle, the animal forcibly raises his
fore-limbs by flexing them. Flexion is produced to the same extent, and
even more so, and for a longer period, in felides which crouch.
In extension, on the contrary, the forearm is carried backward. This
movement being limited only by the contact of the tip of the olecranon
with the bottom of the olecranon fossa of the humerus, the forearm is
enabled, in this case, to move until it is in line with the arm. For
example, during walking, when one of the anterior limbs, having reached
the end of its resting stage, is considerably inclined downwards and
backwards.
The apex of the olecranon process--that is to say, the point of the
elbow--forms a marked prominence, more salient in flexion than in
extension, as in the corresponding region of the human elbow.
=The Radio-ulnar Articulation.=--It is in the dog and the cat, in which
the two bones of the forearm articulate by their extremities only, and
remain separate in the rest of their extent, that the articulations call
for special notice.
In the upper part, the radius rotates on itself; while below, it rotates
around the ulna. It follows that the forearm, which in all quadrupeds is
in a state of permanent pronation, can, in carnivora, take the position
of supination, or rather, of demi-supination. In fact, whatever be the
mobility of the two bones of the forearm, the movement is not able to
bring the palmar surface to the front, but only to direct it towards the
median line.
=The Articulation of the Wrist.=--Here are found, as in man, three
superimposed articulations: the radio-carpal, intercarpal, and
carpo-metacarpal.
If we remember the movements which take place at the plane of these
articulations in man, and take account of the fact that the mobility of
the limbs is reduced just in proportion as they are simplified in
structure so as to become organs of support only, we can easily
comprehend that, in the horse and the ox, and, in a word, animals that
have a canon bone, the movements of the wrist are little varied in
character, while in carnivorae, on the other hand, they are relatively
more so.
We will remember that in the ox and the horse the region of the wrist is
called the _knee_.
In flexion, the hand is bent backwards; in extension it is carried
forwards. These two movements take place especially in the radiocarpal
and intercarpal articulations. In the first of these articulations, it
is the superior row of the carpus which glides backwards and forwards on
the corresponding articular surface of the forearm. In the second
articulation, it is the second row which moves; gliding on the inferior
articular surfaces of the row above it. This inferior row carries the
metacarpus with it; for the carpo-metacarpal articulation is much less
mobile than either of the other two.
In flexion, the articular surfaces are separated from one another in
front; and the changes of form which result from this are noticeable on
the anterior surface of the 'knee.' Moreover, at that moment this region
contrasts markedly in its outlines with the parts above it and below
it--that is to say, with the corresponding surfaces of the forearm and
of the canon bone.
As for the lateral movements, by which the hand is inclined outwards and
inwards in its movements at the wrist, they exist to an appreciable
extent in the cat and the dog only; in order to understand this, it is
enough to compare the shape of the articular surfaces of this region in
carnivora and the horse, for example. In the latter, those surfaces are
almost plane; in the cat, on the contrary, they are curved (inferior
surface of the forearm, concave; superior border of the carpus, convex).
These latter, then, are, in form, similar to those which exist at the
same level in the human being; this explains the possibility of
analogous movements of the whole hand--that is to say, of the movements
of abduction and adduction.
=The Metacarpo-phalangeal Articulations.=--With regard to the mobility,
it is in these articulations, as in those of the wrist--that is to say,
although in all quadrupeds the first phalanges can be flexed and
extended on the metacarpus, it is only in the cat and dog that lateral
movement is possible. Indeed, in the horse, in which the principal
metacarpal terminates inferiorly in two convex surfaces, which are
separated by a crest; and where the whole articulates with a cavity on
the superior extremity of the first phalanx; because of the hinging of
these surfaces, there can only be movements of opening and closing of
this articulation. The first phalanx is directed backwards during
flexion and forwards during extension. In the dog and the cat the digits
can be separated from each other, and also drawn together--that is to
say, abducted and adducted; but, as in man, these movements can be made
only when the first phalanges are in the state of extension. During
flexion they are impossible, because of the tension of the lateral
ligaments, which increases as the flexion is more pronounced. This can
be demonstrated, for example, in the cat, which, in order to separate
the digits, opens the hand widely--that is to say, forcibly raises the
first phalanges.
=The Interphalangeal Articulations.=--The phalanges are in contact with
one another by surfaces, which, on one side, are of trochlear form, and,
on the other, are moulded on these trochleae; accordingly, at the level
of these articulations, the movements of flexion and extension only can
take place.
In the felidae, the claws which the third phalanges bear cannot be
utilized when the latter are in a state of extension, at which time,
being forcibly raised, they are, in fact, placed on the outer sides of
the phalanges, which are grooved to receive them. But when the animal
wishes to use them, it flexes those third phalanges, of which the
terminal extremity is then projected forward, and the claws are ready to
fulfil their function. But at the same time it extends the first
phalanges, to produce a certain tension of the flexors of the digits,
and thus enable the latter to act with greater efficacy, with a minimum
of contraction. We can demonstrate this action experimentally on
ourselves. It is enough to carry the first phalanges forcibly into a
state of extension; the third phalanges then become flexed, quite
spontaneously, by the tension of the tendons of the flexors which are
inserted into them.
At the same time, if we examine the felidae which we have taken as
examples, when the first phalanges are in the state of extension, the
digits will be found to be separable, as we have already indicated in
connection with the metacarpo-phalangeal articulations, with the result
that the claws are then able to lacerate a wider surface.
The extension of the ungual phalanx, which determines the retraction of
the claw and stops its action, is the mechanical result of an elastic,
fibrous apparatus which is attached to each of the third phalanges, and
has its origin of the second.
THE POSTERIOR LIMBS[14]
[14] Examine Figs. 21, 33, 34, 38, 39, 49.
The posterior limbs are divided, as are the inferior limbs of the human
being, of which they are the homologues, into four parts: pelvis, thigh,
leg, and foot.
The Pelvis
The pelvis, which incompletely limits the abdominal cavity, inferiorly
in the vertical position of the body and posteriorly in the normal
attitude of quadrupeds, is formed by the iliac bones and sacrum--the
coccyx forming a prolongation of the latter. We have already described
the two latter (pp. 10 and 11) in connection with the vertebral column,
of which they form the inferior or posterior portion or segment,
according to the attitude of the individual.
=The Iliac Bone.=--The iliac or coxal bone, is a paired or
non-symmetrical bone, united below to its fellow of the opposite side,
while it is separated from it above by the sacrum.
In all animals, as well as in man, the iliac bone, at the beginning of
life, consists of three parts, which afterwards unite and fuse together
and join at the middle of the bottom of a deep cavity which is situated
on the outer aspect of the bone--the cotyloid cavity.
Of those three portions when examined in the human iliac bone, that
above the cavity is the ilium; that on the inside is the pubis; and the
last, the lower one, is the ischium. In quadrupeds, the iliac bone
being, in its entirety, directed much more obliquely downwards and
backwards, the relative position of these constituent parts is a little
modified: the ilium is in front, the pubis is still internal, but in a
more inferior position, and the ischium is behind the cotyloid cavity.
We notice this peculiarity of the development of the iliac bone because
it is customary to continue to apply to the osseous regions which
correspond to these parts the names by which they were known when
independent bones.
[Illustration: FIG. 43.--THE LEFT ILIAC BONE OF THE HUMAN BEING:
EXTERNAL SURFACE, PLACED IN THE POSITION WHICH IT WOULD OCCUPY IN THE
SKELETON OF A QUADRUPED.
1, Cotyloid cavity; 2, ilium; 3, iliac crest; 4, anterior iliac crest;
5, posterior iliac spine; 6, pubis; 7, tuberosity of the ischium; 8,
obturator foramen; 9, ischiadic spine.]
[Illustration: FIG. 44.--LEFT ILIAC BONE OF A QUADRUPED (HORSE):
EXTERNAL SURFACE.
1, Cotyloid cavity; 2, ilium, external iliac fossa (directed upward in
the horse); 3, iliac crest; 4, anterior iliac spine (directed inwards in
the horse, it is the angle of the haunch); 5, posterior iliac spine
(directed inwards in the horse; it is the angle of the haunch); 6,
pubis; 7, tuberosity of the ischium; 8, obturator foramen; 9, ischiadic
spine, or subcotyloid foramen.]
The bones which form the skeleton of the pelvis of quadrupeds are
proportionally more elongated and less massive than those of the human
pelvis (Figs. 43 and 44).
We find, on the external surface of the iliac bone, the cotyloid cavity,
whose border is interrupted by the cotyloid notch; a deep notch which
looks downwards.
In front of this cavity is the ilium. This portion, narrow in the part
which is next the cavity, is directed forwards and upwards, expanding
more and more as it passes upwards. It presents an external or superior
surface (external in some animals, superior in others), which recalls
the external iliac fossa; and an internal or inferior surface, at the
superior part of which is found the auricular surface for articulation
with the sacrum.
The anterior border of the ilium is rough; this is the iliac crest, at
the extremities of which we find, below or outside, a prominence which
corresponds to the anterior superior iliac spine of man; and internally
another projection which corresponds to the posterior iliac spine.
Immediately above the cotyloid cavity is a rough crest, which is known
as the _supracotyloid crest_, which is, however, no other than the
homologue of the sciatic spine. In front of this prominence, the border
of the ilium, which is notched, forms the great sciatic notch.
If, still taking the cotyloid cavity as the point of departure, we
proceed inwards--that is, towards the median line of the body--we find
the pubis; if in a posterior direction, the ischium. These two portions,
pubis and ischium, limit an oval orifice, the subpubic foramen.
In the human skeleton, the pubis of one side is united to that of the
opposite side, to form the pubic symphysis. In the animals which we are
now studying a portion of the ischium enters into the formation of the
symphysis; in other words, it is formed, not only by the body of the
pubis, but also by the descending branch or ramus of the pubis and a
portion of the ascending branch or ramus of the ischium, which are fused
with those of the opposite side. It results that, though in the human
being the symphysis is short and the ischio-pubic arch large, in
quadrupeds it is the opposite. In them the arch is a mere slot, and
being formed by the ischium alone, merits the name of the ischial arch.
The ischio-pubic symphysis is very large, and forms a horizontal surface
relatively extensive, a sort of floor, on which rest certain organs
which occupy the cavity of the pelvis.
The posterior and external angle of the ischium is rough and prominent;
it is the tuberosity of the ischium. This forms a projection under the
skin; it also does in man when the trunk is strongly inclined forwards,
while the thighs are maintained in the vertical position. In
marsupials--opossum, kangaroo, and phalanger--the pelvis at its pubic
region is surmounted by two bones, situated one on each side of the
median line, and arranged in the form of a fork of two prongs (Fig. 45).
These, which are called _marsupial bones_, support the pouch which, in
animals of this genus, lodges their young, which, at the time of birth,
are incapable of supporting a separate existence, their development
being absolutely incomplete.
[Illustration: FIG. 45.--PUBIC REGION OF THE PELVIS OF A MARSUPIAL
(PHALANGER, FOX).
1, Symphysis pubis; 2, obturator foramen; 3, marsupial bones.]
In the cetaceans--for example, the dolphin--because of the absence of
posterior limbs, the pelvis is represented by two separate bones only,
which have no connection with the vertebral column. In birds, the pelvis
is remarkable for its elongated form (see for its form Fig. 21, and for
details Fig. 46). The cotyloid cavity is pierced by an opening, and
presents on its posterior border, which is here a little prominent, a
surface with which the great trochanter is in contact.
The ilium is very highly developed, and is fused in the median line with
the ilium of the opposite side, the last dorsal vertebrae, the lumbar
vertebrae, and the sacrum. Because of these relations with the dorsal
vertebrae, it is in contact anteriorly with the last ribs, which
consequently emerge from each side of the iliac region of the pelvis.
The ischium forms a plate of bone which, in part, closes the external
portion of the cavity of the pelvis. Its superior border is separate for
a certain distance from the external border of the ilium; there is thus
left an opening of more or less considerable size, which represents or
takes the place of the great sciatic notch.
[Illustration: FIG. 46.--PELVIS OF A BIRD (THE COCK): EXTERNAL SURFACE,
LEFT SIDE.
1, Ilium; 2, ischium; 3, pubis; 4, inferior extremity of the pubis; 5,
sciatic foramen; 6, oval foramen, homologous to the obturator; 7,
coccygeal vertebrae.]
The pubis, long and slender, is in connection with the inferior border
of the ischium, of which it follows the general direction; and
circumscribes with this latter, below the cotyloid cavity, an oval
orifice, which is the homologue of the obturator foramen. Its inferior
extremity reaches beyond the corresponding part of the ischium, bending
towards the middle line, but without joining the pubis of the opposite
side. On this account there is no symphysis pubis in birds.
Nevertheless, an exception must be noted in the case of the ostrich, the
pubic bones of which meet in the middle line, and are articulated in
form of a symphysis.
The Thigh
A single bone, the femur, forms the skeleton of this portion of the
lower limb.
=The Femur.=--The bone of the thigh is, in man, directed downwards and
inwards; this obliquity, we may remind the reader, is due to the
difference in length of the two condyles which form its inferior
extremity; the internal is the more prominent, the result of which is
that when the femur is held vertically, the internal condyle descends
lower than the external. Now, as those two articular expansions rest on
the horizontal plane formed by the upper extremity of the tibia, it
follows that the superior part of the femur inclines towards the side of
the shorter condyle--that is to say, outwards--and that, the leg being
vertical, it and the bone of the thigh unite in forming an angle, of
which the apex is directed towards the inner side of the knee.
In many mammals the two condyles are equally prominent, the result of
which is that the femur inclines neither inwards nor outwards, but is
contained in a plane parallel to the axis of the trunk; while the leg is
included in the same plane. Nevertheless, although contained in the
plane which we have just indicated, the femur is obliquely placed, and
directed downwards and forwards; it accordingly forms, with the pelvis,
an angle, of which the opening is directed to the anterior aspect of the
body.
In reptiles and in birds the femur and leg are both placed in the same
plane, but this plane is not parallel to the axis of the trunk. This is
the result, on the one hand, of the thorax being wide, and, on the other
hand, of the femur, which is directed forwards, being in contact by its
anterior extremity with the lateral aspect of the costal region, it is
thus necessarily placed in a direction forwards and outwards, and the
knee is further removed from the axis of the trunk than is the
articulation which unites the thigh with the pelvis.
The femur, like the humerus, is almost completely enveloped by muscular
masses, which bind it to the lateral walls of the abdomen. Its inferior
extremity alone is free, and is always the more so in proportion to its
elongation--that is to say, as it belongs to an animal whose foot is
more divided. The femur in this respect conforms to the law which we
have indicated in connection with the bone of the arm, in which the
development, as to length, is in proportion to the division of the hand.
If we compare the femur of certain animals with that of man, we see that
the corresponding details of form are readily recognisable, but they are
slightly modified. Thus, on examining the superior extremity, we find
there a head, a neck, a great trochanter, and a lesser; but the neck is
usually short and thick, and the great trochanter does not occupy the
same level with regard to the articular head of the bone. In man, the
great trochanter does not rise to the level of the head of the femur; in
the dog and the cat it approaches that level; in the horse and in
ruminants it rises above it.
With regard to the inferior extremity, its surfaces undergo
modifications which are further accentuated as we pass from the
digitigrades to the ungulates, or unguligrades. We know that in man the
femoral trochlea is continuous behind, without interruption, with the
condyles--that is to say, that each of the condyles is the continuation
of one of the lips of the trochlea. We have just said that the trochlea
is continuous without interruption with the condyles; this is accurate.
Nevertheless, we must remark that, at the level of the junction of these
surfaces, the bone presents a slight constriction, which is more marked
on the external than on the internal aspect. This constriction, which is
but slightly marked in man, is accentuated in the dog and the cat; in
the ruminants and the solipeds it is still more pronounced so that we
may say that in these latter the trochlea and the condyles are almost
completely separated.
There is another modification in regard to the prominence and extent of
the two lips of the trochlea. In man, the external lip of the trochlea
reaches higher than the internal, and it is more prominent in front. In
the dog, these lips are equal with regard to thickness, but the external
still reaches higher than the internal; in the cat, they are equal in
every respect; in ruminants and solipeds the internal lip is wider,
thicker, and rises higher than the external.
In animals the trochlea is, as a general rule, narrower than in man, and
the condyles are more prominent posteriorly; so that, when viewed from
one of the lateral aspects, the inferior extremity of the femur is, in
them, better developed in the antero-posterior direction.
In birds, the femur is shorter than the bones of the leg; its great
trochanter is in contact with a prominence which occupies the posterior
part of the border of the cotyloid cavity. Instead of articulating at
the level of the knee, with the knee-cap and tibia only, as in man, it
articulates, in addition, with the superior extremity of the fibula. A
similar arrangement is found in marsupials and reptiles.
=The Knee-cap.=--This bone, developed in the thickness of the tendon of
the triceps muscle of the thigh, is in contact, by its posterior
surface, with the femoral trochlea. The two articular surfaces which are
applied to the lips of the trochlea present, with regard to their
extent, an inequality which is in proportion to the arrangement which we
have above indicated--that is, while in man it is the external surface
which is the larger, in the horse it is the internal. We shall see what
the general form of the knee-cap is when we come, later on, to study
more particularly the posterior limbs of some animals.
The Leg
The skeleton of the leg consists of two bones: the tibia and the fibula.
The tibia is the more internal and the larger of the two; the fibula is
slender, and situated on the outer side, and a little posterior to, the
preceding. The fibula is more or less developed according to the
species; in some it is complete, in others it is very much atrophied.
This peculiarity may be compared with that which we have drawn attention
to regarding the development of the ulna; but here the seriation is less
distinct. Not only in the different species, but even in the individuals
of the same species, the development of the fibula presents little
regularity. In quadrupeds, the bones of the leg are directed obliquely
downwards and backwards, so that they form, with the femur, which is
directed obliquely downwards and forwards, an angle, the apex of which
is placed at the anterior surface of the knee.
=Tibia.=--The tibia of quadrupeds is readily comparable with that of
man; as in the case of the latter, its shaft has three surfaces--an
external, which is hollowed out in its upper portion, and becomes
anterior below; an internal, slightly convex and subcutaneous; the
posterior, which presents, in its superior part, a crest, the oblique
line of the tibia, and some rugosities. The borders separate the
surfaces. The anterior border, or crest of the tibia, is prominent in
its superior part; below it gradually disappears in passing towards the
internal aspect of the inferior extremity. The external and internal
borders separate the corresponding surfaces from the posterior one.
The superior extremity is thick, and expands in forming three
tuberosities: two lateral and an anterior. The anterior tuberosity,
situated at the superior part of the crest of the tibia, is very
prominent; for this reason the superior extremity is very much expanded
in the antero-posterior direction--hence it results that this diameter
is equal to the transverse, and sometimes even greater. In man, it is
the latter which is the larger. The anterior tuberosity is visible under
the skin.
The inferior extremity, less thick, is prolonged internally by a
prominence which corresponds to the internal malleolus of man. In
animals whose fibula is but slightly developed the tibia presents, on
the external part of its inferior extremity, a small prominence, which
replaces the fibular malleolus. The ruminants must, however, be
excepted, in which we find in this region a special bone, which certain
authors look on as the inferior part of the fibula (see p. 97). The
inferior surface of this extremity of the tibia is articular; and is in
contact with one of the tarsal bones, the astragalus. Because the
superior surface of this latter has the form of a pulley, a pulley much
more marked than that on the human astragalus, the corresponding surface
of the tibia, which has the opposite form, presents two lateral
cavities, separated by a median ridge, which is directed forwards and
slightly outwards; this ridge projects into the groove of the pulley.
=The Fibula.=--This bone, situated at the back of the external
surface of the tibia, is, as we have said, more or less developed. Its
superior extremity, or head, articulates with the external tuberosity
of the tibia. Its inferior extremity, when it exists--it is this
which disappears in animals which have the fibula incompletely
developed--forms a prominence which, placed on the external surface of
the inferior extremity of the tibia, articulates with the astragalus,
and recalls the external malleolus of man.
We have stated above that it is the inferior extremity of the fibula
which disappears when the bone is incompletely developed; it is
necessary to except the bat, in which the fibula, fairly well developed
at its inferior extremity, by which it articulates with the tibia, thins
off in its superior portion, and does not reach the corresponding
extremity of the latter. Further, as in this animal the surface of the
knee, which corresponds to the anterior surface of the same region in
other animals, is turned backwards, the result is that the fibula is
situated on the inner side of the tibia, instead of being placed on the
outer.
The Foot
The foot, in animals, as well as in man, is formed of three portions,
which, as we pass from the part which articulates with the leg towards
the terminal extremity, are: the tarsus, the metatarsus, and the toes.
These three portions are the homologues of the carpus, the metacarpus,
and the fingers, which, as we have already seen in the case of the hand,
are the osseous groups which form its skeleton. The tarsus is formed of
short bones, as the carpus is; these are, in man, seven in number. The
bones are arranged in two rows: one, the posterior, formed of two bones
superimposed--the astragalus, by which the tarsus articulates with the
leg, and the calcaneum, which forms the prominence of the heel; and an
anterior row formed of five juxtaposed ones--the cuboid, situated
externally, and the scaphoid internally, in front of which are found the
three cuneiforms. To the tarsus succeeds the metatarsus, whose form
reminds us very much of that of the metacarpals.
With regard to the toes, which we enumerate in proceeding from the most
internal to the most external, they are formed of phalanges, which are
three in number for the four outer toes; but the number is reduced to
two in the case of the first--that is, the so-called great-toe.
The bones of the tarsus are not seven in all animals; they are fewer in
ruminants and solipeds. We already know that, in the latter, the
metacarpals and the digits are equally reduced in number; the same is
the case for the metatarsals and the toes. We will analyze these
differences when dealing with the species individually.
When we studied the anterior limbs, we saw in passing from the
plantigrades to the digitigrades, and finally the ungulates, or
unguligrades, as the hand became hyperextended, the carpus was raised
and more and more removed from the ground. We shall establish the
existence of the same condition in the posterior limbs; in the
plantigrades the tarsus rests on the ground; in the digitigrades it is
removed from it; while in the unguligrades the distance which separates
it from the point of support is still more considerable; and it is,
indeed, necessary to imagine that if these latter were plantigrades,
would occupy the position on the ground which is indicated by Fig. 47.
In veterinary anatomy the tarsus is called the _ham_; a name we adopt
in conformity with usage, but which we cannot but regret, as in human
anatomy the ham is the region of the posterior surface of the knee.
The general arrangement of the region of the digits of the posterior
limbs in birds, presents some points of interest.
We shall merely say with regard to the metatarsus, that it is formed by
a single bone, which in the cock is furnished towards its inferior third
with a pointed process, the _spur_. At the inferior part, there is,
however, found another, which is but very slightly developed, and with
which the first phalanx of the innermost toe articulates.
The toes are, in the majority of species, four in number:[15] an
internal, which is directed backwards, and corresponds to the great-toe;
the others are directed forwards. This arrangement is constant in
grallatores (wading birds), gallinaceae[16] (domestic fowls), and
raptores (birds of prey).
[15] In spite of the fact that the custom is to designate the terminal
portions of the foot of birds by the name of digits, we prefer to
employ here the terms _foot_ and _toes_. In adopting this
decision we believe we are acting according to a more didactic
method. Homology of names should, in our opinion, always
accompany homology of regions.
[16] With regard to the gallinaceae, we must add that in certain
varieties the number of toes is five; those which are directed
forwards are three in number; the internal one which passes
backward, is double. The two toes which are the subject of this
special arrangement are placed very close together, and are
nearly always superimposed. This condition is found in the Houdan
and Dorking breeds.
[Illustration: FIG. 47.--POSTERIOR LIMB OF THE HORSE PLACED IN THE
POSITION WHICH IT SHOULD OCCUPY IF THE ANIMAL WERE A PLANTIGRADE: LEFT
LIMB, EXTERNAL SURFACE.
1, Tibia; 2, astragalus; 3, calcaneum; 4, metatarsus; 5, first phalanx;
6, second phalanx; 7, third phalanx.]
In climbing birds (parrots, woodpeckers, and toucans), the innermost
toe is not only directed backward, but the external toe accompanies it
in that direction; consequently, there are two posterior and two
anterior toes. Sometimes they are all directed forwards; this
disposition is found in the martins. In some birds, the number of toes
is reduced to three: the cassowary shows this reduction; in others, the
number is still further diminished--the ostrich, for example, has but
two.
[Illustration: FIG. 48.--SKELETON OF THE FOOT OF A BIRD (THE COCK): LEFT
SIDE, EXTERNAL SURFACE.
1, Metatarsus; 2, spur; 3, rudimentary metatarsal; 4, first toe; 5,
second toe; 6, third toe; 7, fourth toe.]
Further, we find that, in general, the number of the phalanges
increases, when we examine the toes in commencing with the most internal
(Fig. 48): this has two; then the following one three; that which comes
next in order has four; and the most external toe has five. The
phalanges of this last are short; so that, although it is formed by a
larger number of bones, it is not the longest of the toes.
THE POSTERIOR LIMBS IN SOME ANIMALS.
=Plantigrades=: =Bear= (Fig. 33, p. 50).--The external iliac fossa is
very deep. The femur is longer than the bones of the leg; the great
trochanter does not reach the level of the head of the femur. The
fibula is well developed; it is united to the tibia at its superior and
inferior extremities only.
The foot, which, as in the case of the hand, rests on the ground by the
whole extent of its plantar surface, presents five toes; the shortest of
these is the internal--that is, the toe which corresponds to the
great-toe in man; the third and fourth are the longest, and they are
almost equal; there is a very slight difference in favour of the fourth,
which is slightly superior in dimensions to the third.
=Digitigrades=: =Cat=, =Dog= (Fig. 34, p. 52).--The external iliac
fossa, which looks outwards, is deep; the iliac crest is convex
anteriorly, the convexity is continued from one iliac spine to the
other.
[Illustration: FIG. 49.--PELVIS OF THE DOG, SEEN FROM ABOVE.
1, Iliac crest; 2, external iliac fossa; 3, sacrum; AA', bi-iliac
diameter; BB', bi-ischial diameter.]
In the dog, the distance which separates the anterior iliac spines is
less than that which separates the ischia (Fig. 49). On a skeleton which
we measured, the transverse diameter, the distance from the anterior
iliac spine of one side to that of the opposite side, was 8 centimetres,
whilst the distance which separated the ischia was 105 millimetres; on
another skeleton, the first measurement was 127 metres, and the second
was 146 millimetres. It seems to us unnecessary to multiply examples.
In the cat, the iliac spines are but slightly marked; the result is
that the iliac crest is almost confounded with the inferior and superior
borders of the ilium. The two diameters referred to above are almost
equal (Fig. 50).
We draw particular attention to what we have just noted in regard to the
transverse proportions of the iliac and ischiatic regions of the dog and
the cat. These relations are evidently of importance with regard to
shape, since the iliac crests and the ischia are noticeable beneath the
skin.
[Illustration: FIG. 50.--PELVIS OF A FELIDE (LION), VIEWED FROM ABOVE.
1, Iliac crest; 2, external iliac fossa; 3, sacrum; AA', bi-iliac
diameter; BB', bi-ischial diameter.]
In the dog, the shaft of the femur is slightly convex in front; but in
the cat it is straight. The borders of the shaft are slightly marked, so
that it is almost cylindrical. The _linea aspera_, less prominent than
in man, gains in width what it loses in elevation; it constitutes what
may almost be called a rough _surface_. This surface is narrower in its
middle portion than at its extremities, where it bifurcates to go
upwards to the two trochanters, and downwards to the two condyles. At
the superior extremity, the neck is short, the great trochanter reaching
almost to the level of the head of the femur; the digital _cavity_,
which is situated on the internal surface of the great trochanter, is
very deep. At its inferior extremity it projects strongly backward. The
trochlea is narrow; in the cat its two lips are equally prominent, while
in the dog the external is a little more elevated than the internal,
which on its part is a little thicker. The trochlea is still more
independent of the condyles than in the human femur; it is separated
from these latter by a slight constriction.
The knee-cap is long and narrow.
The tibia of the dog is slightly curved from before backward: it has the
form of an elongated S; this conformation is in great part due to the
very marked projection of the anterior tuberosity and of the superior
portion of the crest, which, a little below that tuberosity, turns
abruptly backwards, and thus describes a curve the concavity of which is
directed forward. The superior part of the external surface is very much
hollowed out.
The superior extremity is much thicker than the inferior one. It is not
only wide in the transverse direction, but is more especially extended
from before backwards; the prominence of the anterior tuberosity is the
cause of the elongation of this antero-posterior diameter. On the
posterior part of the external tuberosity is found a surface to which
the superior extremity of the fibula is applied.
The inferior extremity presents an articular surface, which is formed of
two lateral cavities, separated by a crest, which is directed obliquely
forwards and outwards. The internal part is prominent, and forms the
internal malleolus.
With regard to the fibula, it is united to the tibia by its extremities
and by the inferior half of its shaft. This latter is more expanded
below than in its upper part. The superior extremity is flattened from
without inwards. The inferior extremity projects beyond the articular
surface of the tibia, and forms the external malleolus, which, instead
of, as in man, descending further than the tibial malleolus, stops at
the same level, and even descends a little less than does the latter.
In the cat, the curve of the tibia is less pronounced; this is due to
the fact that the crest, instead of being concave in its middle portion,
is slightly convex anteriorly. The fibula, less flattened than that of
the dog, is united to the tibia by its extremities only, and is separate
in the rest of its extent.
The bones of the tarsus are seven in number, and arranged as in man,
with this difference (which is easily comprehended), that their general
relations are changed on account of the vertical direction of the
tarsus. For example, the astragalus, instead of being above the
calcaneum, is situated in front of it; the cuneiform bones, instead of
being situated in front of the scaphoid, are found below it, etc.
These animals have but four well-developed metatarsals; that which
corresponds to the great-toe is represented merely by a small
style-shaped bone, situate at the internal part of the region.
Nevertheless, we find this toe fully developed in some dogs.
Notwithstanding this, the bones which form it are, however, but
rudimentary, and much smaller than those of the innermost digit of the
fore-limb.
Sometimes it is double; this condition is demonstrable in individuals
belonging to breeds of large size. The median metatarsals are more fully
developed than the other bones of the same region which are next them.
Viewed as a whole, the metatarsal bones are a little longer than the
metacarpals; the result is that the distance which separates the tarsus
from the ground is a little greater than that which separates the carpus
from the plane on which the anterior limbs rest. The length of the
calcaneum still further exaggerates this difference, and, as in the
animals with which we shall occupy ourselves later on, the projection
which this bone forms is distinctly higher than that which is produced
by the pisiform.
The metatarsus, as a whole, is a little narrower than the metacarpus;
not only on account of the presence of a thumb in the anterior limb,
but, further, because the bones of this latter region are wider than
those of the corresponding part of the posterior limb.
The phalanges closely resemble those of the anterior limbs.
=Unguligrades=: =Pig= (Fig. 38, p. 58).--The pelvis in this animal
presents a few of the characters which we shall again meet with in the
ruminants and the solipeds; however, the posterior (or internal) iliac
spines are relatively more widely separated from one another than in the
latter. This arrangement reminds us of that found in the carnivora.
The femur presents nothing very special. The knee-cap is thick, and
ovoid in outline.
The fibula is completely developed, as in the carnivora; and is
connected with the tibia at both its extremities.
The tarsus consists of seven bones. The astragalus and the calcaneum
differ slightly from those of ruminants.
The foot, like the hand, has two median digits which rest on the ground
by their third phalanges; and an internal and an external digit, which
are removed from it. The metatarsals are a little longer than the
metacarpals.
[Illustration: FIG. 51.--PELVIS OF THE OX: SUPERIOR SURFACE.
1, Iliac crest; 2, external iliac fossa; 3, sacrum; AA', bi-iliac
diameter; BB', bi-ischiadic diameter.]
=Unguligrades=: =Sheep=, =Ox= (Fig. 39, p. 61).--The pelvis of ruminants
of this group closely resembles that of the horse, which we will study
later on (see p. 99). That which we must at once point out is that, with
regard to the ratio formed by a comparison of the bi-iliac and
bi-ischiatic diameters, it may be placed between the ratio obtained in
comparing those diameters in the pelvis of the carnivora and that of the
solipeds. Indeed, in the ruminants, the distance which separates the
ischia exceeds the width of one iliac only, and does not equal, as in
the felide, the total width of the anterior part of the pelvis (Fig.
51). In the skeleton of the ox, which forms part of the anatomical
museum of the Ecole des Beaux-Arts, the bi-ischiadic diameter is 39
centimetres, whilst the width of one iliac crest is 29 centimetres, so
that, in contrast to that which we find in the dog, the width of the
ischiadic region is less than that formed in front by the addition of
the iliac crests.
The great trochanter is large, and extends beyond the level of the plane
in which the head of the femur is found.
In the ox, the linea aspera, instead of being a narrow crest, is spread
out, and forms in reality a surface; the posterior surface of the femur.
At the inferior and external part of this surface is situated a cavity
which surmounts the corresponding condyle, and is known as the
_supracondyloid fossa_. On the internal part of the same region there
are a series of tubercles, which, because of their position in relation
to the corresponding condyle, constitute the _supracondyloid crest_.
The internal lip of the trochlea is much thicker and much more prominent
than the external.
The details which we have just now examined in connection with the ox
are less marked in the sheep.
The trochlea, narrow as a whole, is clearly separate from the condyles
by a very marked constriction.
The patella, which is thickened in the antero-posterior direction, has
the shape of a triangular pyramid with the base upwards. Its posterior
surface, which articulates with the trochlea, presents an arrangement
which is adapted to the disposition of this latter--that is to say, the
surface which is in contact with the internal lip is larger than that
which articulates with the lip of the opposite side.
The tibia of the ox is proportionately shorter than that of the sheep.
The shaft of this bone is flattened from before backwards, in its
inferior half. The median crest of the articular surface of the inferior
extremity is the most prominent part of that region.
[Illustration: FIG. 52.--TARSUS OF THE OX: POSTERIOR LEFT LIMB,
ANTERO-EXTERNAL SURFACE.
1, Tibia; 2, coronoid bone of the tarsus; 3, superior articular surface
of the astragalus; 4, inferior articular surface of the astragalus; 5,
calcaneum; 6, cuboido-scaphoid bone; 7, great cuneiform bone--the small
cuneiform bone is situated at the back of the latter; 8, principal
metatarsal--the small, or rudimentary, metatarsal bone is very small; it
is situated at the back of the preceding, and is not to be seen in the
sketch. It would be visible if the view were directly lateral, but then
the superior and inferior articular surfaces of the astragalus would be
less apparent.]
The fibula is extremely atrophied. The shaft and superior extremity of
this bone are represented merely by a simple ligamentous cord, which is
sometimes ossified. There remains of the fibula, as a portion well and
distinctly developed, the inferior extremity only. This presents itself
under the form of a small bone situated in the region ordinarily
occupied by the inferior extremity of the outer bone of the leg--that is
to say, the external part of the inferior extremity of the tibia; this
little bone articulates with the astragalus and the calcaneum. Some
authors consider it to be a tarsal bone, and describe it under the name
of the coronoid bone of the tarsus (Fig. 52, 2). It is not, perhaps,
quite legitimate to describe it as a bone of this region, for it has
not a homologue in the tarsus of other animals. Its external surface is
rough; its superior border is furnished with a small pointed process
occupying a depression which is provided for it by the tibia. It reaches
lower down than the latter, and forms in this way a sort of external
malleolus, which frames, on the outer aspect, the mortise in which the
astragalus is maintained.
The tarsus, as a whole, has an elongated form; it is formed of five
bones: the astragalus, calcaneum, cuboid and scaphoid, which coalesce,
to form a single bone, and two cuneiform bones, which correspond to the
second and third cuneiform bones of the human foot. These cuneiforms are
called, from their size, commencing internally, by the names small and
great cuneiform.
The calcaneum is long and narrow; it is longer than that of the horse;
it is on the anterior and external part that the bone (coronoid tarsal
bone) which represents the inferior extremity of the fibula is situated.
It forms the prominence known as _the point of the ham_, a prominence
which is no other than the heel, which, in the unguligrades, is, as we
have already said, very far removed from the ground.
The astragalus, which is elongated in the vertical direction, has three
articular surfaces disposed in the form of trochleae: a superior
trochlea, which is in contact with the skeleton of the leg, and which is
present in all animals; an inferior, which replaces the articular head
found on the anterior aspect of the astragalus in man; this articulates
with the portion of the scaphoido-cuboid that corresponds to the
scaphoid; and, lastly, a posterior trochlea with which the calcaneum
articulates. Of these three trochleae, the superior is the most strongly
marked. Between this latter and the inferior is found, on the anterior
surface of the astragalus, a deep depression, which, during flexion of
the foot on the leg, receives a prominence which the inferior extremity
of the tibia presents in its median portion.
We can easily recognise the trochleae which we have been discussing, in
the little bones which children use 'to play at bones'; these bones are
no other than the astragali of sheep.
We have already mentioned that the scaphoid and the cuboid are
ankylosed; they form by their union an irregular bone, on which the
astragalus and calcaneum are supported.
The cuneiforms articulate with the internal half of the superior
extremity of the principal metatarsal; the external half of this
metatarsal articulates with the portion of bone which represents the
cuboid.
The metatarsus is represented by a principal metatarsal, formed by the
coalescence of two metatarsals; we also find in this region a very small
rudimentary metatarsal.
The metatarsus is a little longer than the metacarpus; its transverse
measurement is a little less; on the other hand, it is a little thicker
in antero-posterior direction; from these two differences it results
that the body of the metatarsus is quadrilateral, whereas the metacarpus
presents only an anterior and a posterior surface.
The rudimentary metatarsal is a very small roundish bone, situated at
the back of the superior extremity of the principal metatarsal.
The phalanges closely resemble those of the anterior limbs;
nevertheless, the first and second phalanges differ from the latter in
the fact that they are a little longer and narrower.
At the back of the metatarso-phalangeal articulations, as in the
corresponding region of the anterior limbs, are found the sesamoid
bones. Such also exist at the articulations of the second and third
phalanges.
=Unguligrades=: =Horse= (Fig. 40, p. 64).--The pelvis of the horse
presents a general form which sharply differentiates it from that of the
carnivora; in fact, the ilium is twisted in such a way that the external
iliac fossa does not look outwards, but upwards. It results from this
twist that the anterior iliac spine, which we have seen to be directed
downwards in the carnivora, has become external; and this prominence is
much farther removed from the vertebral column than in the dog or cat.
On the other hand, the posterior iliac spine, which is directed upwards
in the carnivora, has become internal; it is also placed nearer to the
vertebral column, with the result that the distance which separates this
spine from that of the bone of the opposite side is proportionately
less.
The internal iliac spine, which is conical in shape, and curved upwards,
forms a prominence known as _the angle of the crupper_; the external
iliac spine, thick and provided with tuberosities, forms a
clearly-defined prominence; this is the angle of the _haunch_.
The iliac crest, extending directly from one spine to the other, is
curved, its concavity being turned upwards. The external iliac fossa,
which looks upward, is limited anteriorly by this crest, and is, like
the latter, slightly hollowed. The portion of the bone which connects
the ilium to the region occupied by the cotyloid cavity is extremely
narrow; posteriorly, the bone enlarges again to form the ischial and
pubic portions.
The tuberosity of the ischium, thick and curved upwards, but less so
than in the ox, forms the most prominent part of the posterior border of
the region of the thigh; this projecting portion, so sharply defined in
spare subjects, is known as the _point_ or _angle of the buttock_.
Contrary to what we have indicated in the case of the dog, the distance
which separates the ischiatic tuberosities is inconsiderable in
proportion to that which we find between the external iliac spine of one
side and that of the opposite. The bi-ischiatic diameter does not even
equal the width of one iliac bone measured at the level of its crest
(Fig. 53). On the skeleton of the horse in the Ecole des Beaux-Arts, the
distance which separates the tuberosities of the ischia is 225
millimetres; that between the two spines of each iliac bone is 25
centimetres.
The anterior region of the crupper is thus much broader than that
occupied by the ischia.
The femur is relatively short. Its shaft is rectilinear, and does not
present the anterior convexity which is found on the human femur, and
which we indicated when discussing that of the dog. The shaft of the
bone, instead of being prismatic and triangular, presents four surfaces;
the anterior, internal, and external, almost pass into each other, being
separated one from the other merely by rounded and slightly marked
borders; the posterior surface, which is plane, replaces the linea
aspera, which in the horse, instead of presenting the appearance of a
crest, is considerably widened. The numerous irregularities which this
surface presents give insertion to the muscles which correspond to those
attached to the linea aspera.
Between this posterior surface and the external is found a rough
prominence which curves forward; this was designated by Cuvier the
_third trochanter_; it replaces the external branch of the superior line
of bifurcation of the linea aspera; other authors call it the
_infratrochanteric crest_, because it is situated below the great
trochanter. At the inferior part of the same region is found a deep
fossa, the borders of which are rough; this is the _supracondyloid
fossa_.
[Illustration: FIG 53.--PELVIS OF THE HORSE: SUPERIOR SURFACE.
1, Iliac crest; 2, external iliac fossa; 3, sacrum; AA', bi-iliac
diameter; BB', bi-ischiatic diameter.]
Between the posterior surface and the internal are found: above, the
lesser trochanter, which is long and rough; below, at the level of the
supracondyloid fossa, an equally rough surface known by the name of the
_supracondyloid crest_.
The superior extremity is flattened from before backwards. The neck is
not well marked. The great trochanter is very prominent, and projects
beyond the level of the head of the femur. We divide the great
trochanter into three parts: the summit, which is the most elevated
portion; the convexity, which is situated in front; and the crest,
formed by muscular impressions, situated outside and below the
convexity. The digital fossa is situated behind and below the summit of
the great trochanter. With regard to the lesser trochanter, it is placed
so far down that it really forms part of the shaft of the bone, with
which, besides, we have described it.
On the inferior extremity of the femur are two condyles and a trochlea;
the condyles are clearly separated from this latter by a marked
constriction.
The trochlea is directed with a slight obliquity downwards and inwards;
its internal lip is much thicker and more prominent than the external;
this is, accordingly, a condition exactly the opposite of that which
characterizes the corresponding region of the human femur.
The knee-cap is lozenge-shaped; its superior angle projects upward, and
produces a prominence at the part which corresponds to the base of the
human patella, the part which is here the thickest portion of the bone.
Its anterior surface is convex and rough. Its posterior surface presents
two lateral articular facets, separated by a crest; this surface is in
contact with the trochlea of the femur, and, as it is the internal lip
of the latter which is the more developed, it results therefrom that the
internal articular surface of the knee-cap is larger than the external.
The knee-cap contributes to the formation of the region of the posterior
limb which is called the _stifle_.
The tibia is large in its upper portion; in its inferior part it is
flattened from before backwards. The posterior surface of the shaft
presents an oblique line, below which are found vertical rough lines for
the insertion of muscles. The external surface is hollowed out in its
upper part. The anterior tuberosity of the tibia rises just to the level
of the flat articular surface; it is hollowed in its median portion by a
vertical groove of elongated form, which receives the ligament that
binds the knee-cap to the tibia. The external tuberosity is more
prominent than the internal; in it is found a groove for the passage of
the anterior tibial muscle.
The inferior extremity, flattened from before backwards, presents a
surface which is moulded on the trochlea of the astragalus; the median
crest of this surface is thick, and descends lower posteriorly than the
tuberosities which are situated on the external and internal aspects of
this extremity.
Of the two tuberosities, that which is internal is comparable to the
internal malleolus of man, the one on the outer side forms a sort of
external malleolus; but this latter here belongs to the tibia, and not
to the fibula.
The fibula, in fact, does not reach the inferior extremity of the tibia;
it is a poorly developed bone, elongated and terminating inferiorly in a
point, at the middle of the shaft of the tibia or at its lower third.
Its superior extremity, which is slightly expanded, articulates with the
tuberosity which occupies the outer aspect of the corresponding
extremity of the tibia.
The bones of the tarsus are six in number: the calcaneum and astragalus
form the upper row; the cuboid, scaphoid, and two cuneiforms form the
lower (Fig. 54).
The astragalus has not, as in ruminants, an inferior trochlea for
articulation with the scaphoid; this portion of the bone presents a
surface which is slightly convex. It articulates with the tibia by a
trochlea that occupies not only the superior surface, but also the
anterior. This trochlea, which is directed slightly obliquely downwards
and outwards, has a very pronounced form; its lips, which are extremely
prominent, determine by their anterior part one of the features which we
recognise on the anterior aspect of the _ham_--a feature which is still
more accentuated when the metatarsus (_canon_) is extended on the leg.
On the internal surface of the astragalus is found a tubercle, which
forms a projection in the corresponding region of the ham.
The calcaneum, which is not quite so long as that of the ox, forms by
its summit a prominence which is called _the point of the ham_.
The cuboid is small; the scaphoid is large, and flattened from above
downwards. Of the two cuneiforms, the more external is the larger; it
closely resembles the scaphoid; it is flattened from above downwards as
is the latter; but it is a little smaller in size. The small cuneiform,
which occupies the inner side of the tarsus, is the smallest bone in
this region; it is sometimes divided into two parts; this raises the
number of the cuneiforms to three, and that of the bones of the tarsus
to seven.
[Illustration: FIG. 54.--TARSUS OF THE HORSE: LEFT POSTERIOR LIMB,
ANTERIOR SURFACE.
1, Tibia; 2, internal tuberosity of the inferior extremity of the tibia
(homologue of the internal malleolus of man); 3, external tuberosity of
the inferior extremity of the tibia (homologue of the external
malleolus); 4, median crest lodged in the groove of the pulley of the
astragalus; 5, pulley of the astragalus; 6, internal tuberosity of the
astragalus; 7, calcaneum; 8, cuboid; 9, scaphoid; 10, great cuneiform,
the small cuneiform is placed behind this latter; 11, principal
metatarsal; 12, external rudimentary metatarsal. The internal
rudimentary metatarsal, being more slender than the external, does not
appear in the figure.]
The bones of the metatarsus and the phalanges are equal in number to the
corresponding bones in the anterior limbs; they are formed on a type
analogous to that of these latter. Accordingly, we shall merely indicate
the differences which characterize them.
The principal metatarsal is longer than the metacarpal of the same
class; its shaft is more cylindrical; its inferior extremity is somewhat
thicker. The external rudimentary metatarsal is better developed than
the internal; in the metacarpus the reverse is the case.
The phalanges so far resemble those of the anterior limb that, as
differential characters, we need point out only the following: the first
phalanx of the hind-foot is a little shorter than that of the fore-foot;
its inferior extremity is a little narrower, and its superior extremity
a little thicker. The second phalanx is a little less expanded
laterally.
The difference in appearance which the three phalanges, anterior and
posterior, respectively present are to be borne in mind; for they are
correlated to the general form of the fore and hind feet. We will
establish this point when we come to study the hoof (see Figs. 101 and
102, p. 257). In the fore-foot the ungual phalanx has its inferior
surface limited externally by a circular border, while the same bone of
the hind-foot has this surface a little narrower, more concave, and
limited by two curved borders which unite anteriorly to form an
angle--an arrangement which gives to the general outline of this region
the form of the letter V.
Articulations of the Posterior Limbs
=The Coxo-femoral Articulation.=--The head of the femur is received in
the cotyloid cavity; these are the osseous surfaces in contact in this
articulation. They are maintained in position by a fibrous capsule and a
round ligament. To this latter is found attached, in the horse, a
fasciculus which, commencing, as does the round ligament, at the
depression on the head of the femur, emerges from the cotyloid cavity by
the notch which is present in its circumference, and is attached to the
anterior border of the pubes, to blend with the tendon of the rectus
muscle of the abdomen. This is the pubio-femoral ligament.
The movements which this joint permits are the same in the quadrupeds as
in man, but less extensive. They are: flexion and extension, abduction
and adduction, the two latter being much more limited than the former.
There is also rotation.
By flexion, the inferior extremity of the femur is directed forwards;
the bone of the thigh then takes a more oblique direction than the
normal. This movement takes place, for example, when the animal carries
forward one of its hinder limbs. Extension, which takes place in an
inverse sense, is produced when the foot is fixed on the ground, while
the body is projected forward. It is also produced in the action of
kicking.
As for the lateral movements--viz., abduction and adduction--they are
less extensive than the preceding movements. The absence of the
pubio-femoral ligament in other quadrupeds than the horse explains why
in them abduction is less limited than in the latter. Indeed, it is the
tension of this ligament, occasioned by the abduction of the thighs,
which arrests more quickly the movement in question.
=Articulation of the Knee.=--This articulation, as in man, is formed by
the femur, the patella, and the tibia.
In the horse the ligament of the patella is not single, but consists of
three parts, designated, on account of their position, by the respective
names of external, internal, and median patellar ligaments. The two
former come from the angles on the corresponding borders of the
knee-cap; the median springs from the anterior surface and inferior
angle of the same bone. They all three pass to their termination on the
anterior tubercle of the tibia. The external ligament is the strongest,
and the internal ligament the least developed.
In the dog, the cat, the pig, and the sheep, the patellar ligament
consists of a single band. The articulation is further strengthened on
the sides by lateral ligaments--an internal and an external.
With regard to the principal movements, these are flexion and extension,
to which may be added movements of rotation of limited extent. In
flexion, the leg bends on the thigh; its inferior extremity is directed
upwards and backwards; the angle which the tibia naturally forms with
the femur becomes less obtuse.
But it should be understood that one part of this description--that
which has relation to the leg--holds good only when the femur is in its
normal condition, or in flexion. Indeed, at the close of the movement in
which, during a step, the foot is in contact with the ground--that is,
at the termination of the resting stage--the inferior extremity of the
tibia is directed backwards. But the femur is then in a state of
extension, and in regard to this latter the attitude of the leg is
unchanged.
[Illustration: FIG. 55.--EXTENSION OF THE LEG: RIGHT POSTERIOR LIMB OF
THE HORSE, EXTERNAL SURFACE. (AFTER A CHRONOGRAPHIC STUDY BY PROFESSOR
MAREY.)]
At this moment, notwithstanding the direction, which recalls that which
it has at the time of flexion, the leg is not bent on the thigh; on the
contrary, it is almost in the line of its continuation (Fig. 55). As we
have done in connection with the articulations of the anterior limbs, we
borrow this figure from the interesting chronophotographic studies of
Professor Marey.[17]
[17] E. J. Marey, 'Analysis of the Movements of the Horse by the
Chronophotograph' (_La Nature_, June 11, 1898).
=The Tibio-tarsal Articulation and of the Bones of the Tarsus.=--In the
region which veterinary anatomists call the ham, the articulations of
the leg and foot alone call for special study in the case of the horse.
The articulations of the bones of the tarsus, and of these with the
metatarsus, do not offer any interest with regard to mobility, this
being almost wholly absent at that level.
The leg and the astragalus, in a general way, are placed in contact by
such articular surfaces that the resulting joint, which is a true hinge,
permits movements of flexion and extension only. Indeed, as we have
indicated above, the tibia is furnished, on the inferior surface, with a
crest that fits into the deep groove which is situated on the
corresponding surface of the astragalus.
During flexion, the anterior surface of the foot tends to approach the
anterior surface of the leg, the angle formed by these two segments
becoming more and more narrowed. The displacement in the opposite
direction characterizes extension.
In other quadrupeds, the articulations which bind together the bones of
the tarsus possess a little more freedom of movement. The shape of these
bones, and particularly the shape of the surfaces of the astragalus,
which are in contact with them, allow movements in this region, in the
case of the dog and cat, which, without being so extensive as those of
the human foot, in the subastragaloid articulation, nevertheless, recall
the mobility which we find in the human species at this level--that is
to say, rotation, abduction, and adduction of the foot.
As for the articulations of the metatarsus with the phalanges, and of
the phalanges with one another, they resemble those of the anterior limb
too closely that it should be necessary to study them here. Such a study
would be, in this case, but a repetition (see p. 76, a description of
the articulations in question).
THE HEAD IN GENERAL, AND IN SOME ANIMALS IN PARTICULAR
When we compare, by the examination of one of their lateral aspects, the
skull of man and the same region in other mammals, it is easy to observe
that the relative development of the cranium and face is entirely
different. In the case of man the cranium is large, and the face
relatively small; in animals the face is proportionally much more highly
developed. The measure of the facial angle permits us to note these
differences, and the figures relative to the value of this angle are
sufficiently demonstrative to induce us to indicate those which are, in
a general way, connected with some of the forms in individuals which
here occupy our attention. In the first place, we must remember that the
angle in question is more acute, as the cranium is less developed in
proportion to the facial region (Figs. 56 and 57). It is especially to
this character that we wish to draw attention.
Man 70 deg.-80 deg.
Cat 41 deg.
Dog 28 deg.-41 deg.
Sheep 20 deg.-25 deg.
Ox 18 deg.-20 deg.
Ass 12 deg.-16 deg.
Horse 11 deg.-13 deg.
Besides, in animals the cranium is very prominent superiorly, and the
face, more or less elongated, is sharply projected downwards and
forwards; in man the cranial region occupies not only the superior, but
also the posterior part; the face is short and of a compact form. The
human head, in its general aspect, may be compared to a sphere, while
the skull of the quadrupeds presents the aspect of a quadrangular
pyramid, with the base turned upwards and the summit at the incisor
teeth.
=Direction of the Head.=--Before entering on the study of the bones of
the head, it is necessary, in our opinion, to agree as to the position
in which we shall suppose it to be placed.
The question may seem to be one of little importance; nevertheless, it
cannot be regarded as indifferent, since authors are not all agreed on
this subject.
Some suppose it to be placed vertically--that is, with the incisor teeth
turned directly downwards. Others, on the contrary, suppose it to be
placed horizontally, resting on the whole length of the lower jaw, the
face being then turned upwards. These two extreme methods of arrangement
appear to us to possess inconveniences--at least, for comparison with
the human head.
[Illustration: FIG. 56.--HUMAN SKULL: MEASURE OF THE FACIAL ANGLE BY THE
METHOD OF CAMPER. ANGLE BAC = 80 deg.
The internal wall of the cranial cavity is marked by the dotted line.]
[Illustration: FIG. 57.--SKULL OF THE HORSE: MEASURE OF THE FACIAL ANGLE
BY THE METHOD OF CAMPER.[18] ANGLE BAC = 13 deg.
The internal wall of the cranial cavity is shown by the dotted line.]
[18] We have indicated on this sketch of the skull of the horse the
facial angle measured by the method of Camper, in order that the
correspondence with Fig. 56 may be more complete. But it is
certain that the procedure here employed is in practice not
satisfactory, since the apex of the angle, as we can demonstrate,
is found to be situated within the contour of the head, and that,
consequently, it is rather difficult to localize it precisely in
the case of a given skeleton. Further, because of the absence of
the base of the nose in the complete skull, the auriculo-nasal
line cannot be accurately fixed. It would be the same for most
other animals. This is why the method employed for these latter
is preferably that of Cuvier, or, again, that of Cloquet. In the
former, the apex of the angle of Camper is transferred to the
free border of the upper incisors, but these teeth may be absent,
and, on the other hand, ruminants are destitute of them. In the
second, the same apex is placed at the alveolar border, and the
angle then becomes fairly easy to appreciate.
Indeed, if, when the head is vertical, the same regions of the face
(forehead, nose) are, in the case of animals as well as man, turned
forward, the lower jaw ceases to merit its appellation, as it is then
situated, not below, but behind the upper. Furthermore, if this
position is chosen, for example, for modelling or drawing, it cannot be
obtained without difficulty when we have to deal with an isolated piece
of the skeleton, on account of the absence of equilibrium, which it is
necessary to obviate. It is true that the question of convenience should
not take precedence of all others, and it suffices for us in this
connection to recall, in regard to the human pelvis, that, although the
older anatomists used to represent it as resting commodiously on the
three angles which terminate it at its lower part (ischial tuberosities
and coccyx), this attitude being false, it is customary now to incline
the superior aspect forwards, inasmuch as this arrangement more nearly
conforms to reality, in spite of the fact that it is a little more
difficult so to dispose an isolated pelvis. Further, to return to the
head; if its vertical direction can be demonstrated, for example, in
many horses, it is not sufficiently general to be adopted as the classic
position.
In regard to the facility of placing in position, the horizontal
direction is certainly to be preferred; but this is also far removed
from the natural position in the animal while in the state of repose. On
the other hand, the mind is not satisfied with the idea that certain
regions of the face, such as the nose and the forehead, are then
directed upwards. And yet it is necessary to come to a decision, seeing
that what we are now investigating applies also to the position to which
it is necessary to give the preference in placing the skeleton of the
head when we wish to draw it in profile. That which we adopt is a
compromise, but to us it seems more rational.
The position of the head of the horse, to be normal, should be such as
to give it an inclination of 45 deg. In this case the lower jaw is still
posterior; and, for this reason, we see in adopting this position some
inconveniences from a didactic point of view. Accordingly, we will
suppose the head brought a little nearer to the horizontal, and this,
from the imaginative point of view, has certainly an advantage which we
cannot afford to neglect when addressing artists.
Indeed, let us suppose that to a clay model of a human head we wish to
give the aspect of the head of a quadruped. We should elevate the
occiput; and then, taking hold of the lower part of the face, we should
lengthen it, not in a direction precisely antero-posterior, but
downwards and forwards. It is obviously this latter procedure which, on
the other hand, is carried out when a person wishes to give to his own
face some resemblance to the muzzle of a quadruped.
It is true that, in the position we have adopted, the face is directed
obliquely downwards and forwards, and that there may result a certain
confusion in describing the position of its different parts. On
this account, with the object of not making complications, we
purpose, for the present, to substitute, for example, for the term
'antero-superior'--which when speaking of the position of the forehead
and nose would be more exact--the term 'anterior,' which is sufficiently
comprehensible. The mouth will be, for the same reason, referred to as
being situated at the inferior part of the face, and not the
antero-inferior.
=The Skull.=--The elevation of the cranial region becomes especially
appreciable when we examine the occipital bone. Before verifying this
fact, it is not superfluous to recall the general arrangement which this
bone presents in the human skull. A portion of the occipital bone
occupies the base of the skull; but this base in man is horizontal; to
this region succeeds the shell-shaped portion of the occipital bone,
which, passing vertically upwards, forms with the preceding portion an
angle situated at the level of the external occipital protuberance, and
of the curved line which starts from it on each side. In animals a
portion of the occipital bone is horizontal, it is true; but this bone
being sharply bent at the level of the occipital foramen and condyles,
the result is that the portion which surmounts these latter looks
backwards, and is limited above by the external occipital protuberance,
which forms the culminating point of the skull; this point is situated
between the ears.
[Illustration: FIG. 58.--SKULL OF ONE OF THE FELIDAE (JAGUAR): LEFT
LATERAL ASPECT.
1, Posterior surface of the occipital bone; 2, external occipital
protuberance; 3, condyle of the occipital bone; 4, jugular process; 5,
parietal bone; 6, frontal bone; 7, orbital process; 8, orbital cavity;
9, squamous portion of the temporal bone; 10, external auditory canal,
in front of which is situated the zygomatic process; 11, tympanic bulla;
12, superior maxillary bone; 13, intermaxillary or incisor bone; 14,
nasal bone; 15, anterior orifice of the nasal cavity; 16, malar bone;
17, ungual or lachrymal bone; 18, inferior maxillary bone; 19, condyle
of the inferior maxillary bone; 20, coronoid process; 21, incisor teeth;
22, canine teeth; 23, molar teeth.]
[Illustration: FIG. 59.--SKULL OF THE LION: LEFT LATERAL ASPECT.
This figure is intended to show that in the lion the contour of the face
between the nasal bones and the cranial region is more flattened than in
other felidae, such as the tiger, jaguar, panther, and domestic cat. This
difference is shown by comparison of this figure with the preceding one
(Fig. 58). We are indebted to M. Tramond, the well-known naturalist, for
the indication of this differential character which, from the artistic
plastic point of view, is one of real interest.]
This protuberance, prolonged on each side by the superior curved line of
the occipital bone, is so much the more prominent as this bone bends
sharply a second time, so as to form a third portion, which, looking
forwards, forms part of the anterior aspect of the skull, and proceeds
to articulate with the parietals. On this third portion is found a crest
which, proceeding from the occipital protuberance, is continuous in
front with the parietal crests, to which we will again refer in speaking
of the parietal bones.
On the inferior surface of the human occipital bone are found, at the
level of, and external to, the condyles two bony elevations which bear
the name of _jugular eminences_. They are long in quadrupeds, and
constitute what are designated by some authors the _styloid processes_,
but they must not be confounded with the processes of the same name
which in the case of man form part of the temporal bone. These processes
are very highly developed in the pig, horse, ox, and sheep.
In the ox, the occipital bone is deprived of the protuberance, and is
not bent on itself in the anterior portion, neither does it form the
most salient part of the skull; this latter, which is situated at the
level of the horns, belongs to the frontal bone. In the pig, also, the
occipital bone is not bent upon itself in its anterior portion, but
forms the summit of the head. The occipital protuberance, hollowed on
its posterior surface, rises vertically, and rests upon the parietal
bone, with which it forms an acute angle.
The parietals, two separate bones in the dog and the cat, but fused in
the median line in the ox, sheep, and horse, are of special interest in
regard to the two crests which, in the carnivora, and also in the pig
and the horse, occupy their external surface, and, after diverging from
one another, are continued by a crest which crosses the frontal bone
and ends at the external orbital process of the latter bone.
These crests, known as the _parietal_ or _temporal crests_, recall both
in position and relations the temporal curved line of the parietal bone
of man. They contribute, as in the case of the latter, to the formation
of the boundaries of the temporal fossa.
[Illustration: FIG. 60.--SKULL OF THE DOG: LEFT LATERAL ASPECT.
1, Posterior surface of the occipital bone; 2, external occipital
protuberance; 3, occipital condyle; 4, jugular process; 5, parietal
bone; 6, frontal bone; 7, orbital process; 8, orbital cavity; 9,
external auditory canal, in front of which is found the zygomatic
process; 10, tympanic bulla; 11, superior maxillary bone; 12,
intermaxillary or incisor bone; 13, nasal bone; 14, anterior opening of
the cavity of the nasal fossae; 15, malar bone; 16, lachrymal bone; 17,
inferior maxillary bone; 18, condyle of the inferior maxillary bone; 19,
coronoid process; 20, incisor teeth; 21, canine teeth; 22, molar teeth.]
In the carnivora, these crests are situated, throughout their whole
length, in the median line, the temporal fossae being, accordingly, as
extended as they possibly can be. In certain species, the development of
these crests is such that they form by their union a vertical plate,
which, in separating the two temporal fossae, gives them a greater depth.
In the pig, the parietal crests, analogous in this respect to the
temporal curved lines of the parietal bones of man, are separated by an
interval, proportionately less extended, however, than that of the human
skull. The parietal bone in the ox and the sheep does not enter into the
formation of the anterior surface of the skull; it is formed by an
osseous plate, narrow and elongated transversely, which, with the
occipital bone, constitutes the base of the region of _the nape of the
neck_. It is bent upon itself at the level of its lateral portions so as
to occupy the temporal fossa.
The anterior surface of the frontal bone, which is depressed in the
median line in the dog, but plane in the horse, is limited by two
crests, which, situated on the prolongation of the parietal crests,
diverge more and more from one another in proportion as they occupy a
lower position. This surface terminates externally in two processes,
which are the homologues of the external orbital processes of the human
frontal bone.
The superior border of these orbital processes, situated on the
prolongation of the corresponding parietal crests, contributes to limit
the temporal fossa. Each of these orbital processes terminates in the
following manner: In the bear, dog, cat, and pig, in which the orbital
cavities are incompletely bounded by bone, this process, slightly
developed, is not in connection, by its inferior extremity, with any
other part of the skeleton of the region. In the ox and the sheep, it
articulates with a process of the malar bone. In the horse, it
articulates with the zygomatic process of the temporal bone. The
inferior margin of this process forms a part of the boundary of the
anterior opening of the orbital cavity.
The supra-orbital foramen, which does not exist in carnivora, occupies
in the horse the base of the orbital process. In the ox, it is situated
a little nearer the middle line; and its anterior orifice opens into an
osseous gutter which is directed upwards towards the base of the horn,
while inferiorly it meets the inferior border of the frontal bone; in
the sheep this groove is but slightly developed. In this latter, as in
the ox, it is the frontal bone which forms the most elevated portion of
the skull. In fact, being bent upon itself at a certain level, its
external surface is formed of two planes: one, posterior, which is
inclined downwards and directed backwards; the other, anterior, is also
inclined downwards, but with a forward obliquity. At the union of these
planes the bone forms an elbow, on either side of which are found the
osseous processes on which the horns are mounted.
[Illustration: FIG. 61.--SKULL OF THE PIG: LEFT LATERAL ASPECT.
1, Occipital bone; 2, condyle of the occipital; 3, jugular process; 4,
parietal bone; 5, parietal crests; 6, frontal bone; 7, orbital process;
8, orbital cavity; 9, external auditory canal; 10, zygomatic process;
11, superior maxillary bone; 12, intermaxillary or incisor bone; 13,
nasal bone; 14, anterior orifice of the cavity of the nasal fossae; 15,
malar bone; 16, lachrymal bone; 17, inferior maxillary bone; 18, condyle
of the inferior maxillary bone; 19, incisor teeth; 20, canine teeth; 21,
molar teeth.]
In the bear, the anterior margin of the frontal bone is prolonged by two
small tongues of bone, which, descending on the lateral borders of the
nasal bones, articulate with the superior half of the latter.
The temporal bone is, as in man, furnished with a squamous portion,
from which springs the zygomatic process, which is directed towards the
face, to terminate in the following manner: in the carnivora, the pig,
and ruminants, it articulates with the malar bone by its inferior
border; in the horse, it insinuates itself as a sort of wedge between
the malar bone and the orbital process of the frontal bone, with which
it articulates, as we have already pointed out, and contributes, by a
portion situated in front of this articulation, to form the boundary of
the anterior opening of the corresponding orbital cavity. As in man, the
zygomatic process arises by two roots: one, transverse, behind which is
situated the glenoid cavity of the temporal bone; the other,
antero-posterior, which proceeds to join above with the superior curved
line of the occipital bone.
Behind the glenoid cavity is found the external auditory canal, and,
further back still, the mastoid process. This latter, but slightly
developed in the carnivora, a little more so in the ruminants, and still
more in the horse, has its external surface traversed by a crest, _the
mastoid crest_, which, after becoming blended with the antero-posterior
root of the zygomatic process, proceeds with this latter to join the
superior occipital curved line.
Below the auditory canal is situated a round prominence, highly
developed in carnivora; this is _the tympanic bulla_, also called _the
mastoid protuberance_; it is an appendage of the tympanum.
The Face
The bone of this region, around which all the others come to be grouped,
is, as in man, the superior maxillary. The relations of this maxillary
with the neighbouring bones is not exactly the same in all animals; for
example, in the ox, sheep, and horse, in which the bones of the nose are
wide in their upper part, and in which the lachrymal bone, which is very
highly developed, encroaches on the face, the superior maxillary does
not meet the frontal bone; it is separated from it by the above-named
bones. It unites with it, on the other hand, in the dog and the cat. In
the bear, it is separated from the bones of the nose by a small tongue
of bone which springs from the anterior border of the frontal--a
process which we have noticed in connection with this latter.
[Illustration: FIG. 62.--THE SKULL OF THE OX: LEFT LATERAL ASPECT.
1, Occipital condyle; 2, jugular process; 3, parietal bone; 4, frontal
bone; 5, osseous process, which serves to support the horn (horn-core);
6, orbital cavity; 7, external auditory canal, in front of which is
found the zygomatic process; 8, temporal fossa; 9, superior maxillary
bone; 10, intermaxillary or incisor bone; 11, nasal bone; 12, anterior
orifice of the cavity of the nasal fossae; 13, malar bone; 14, lachrymal
bone; 15, inferior maxillary bone; 16, condyle of the inferior maxillary
bone; 17, incisor teeth; 18, molar teeth.]
In the pig, ox, sheep, and horse, the external surface is traversed, to
a greater or less extent, by a crest which is situated on the
prolongation of the inferior border of the malar bone. This crest, which
is straight in the horse, but curved with its convexity upwards in the
ox and the sheep, is known as _the maxillary spine_ or _the malar
tuberosity_: it gives attachment to the masseter muscle, and, in the
horse, is distinctly visible under the skin. It does not exist in the
carnivora. On the same surface is situated the sub-orbital foramen.
The inferior border is hollowed out into alveoli, in which are implanted
the superior molar and canine teeth. This border is prolonged forwards
from the alveolus, which corresponds to the first molar tooth, to
terminate, after a course more or less prolonged, at the alveolus of the
canine. This space, more or less considerably expanded, which thus
separates these teeth is called the _interdental space_; but this
denomination is not applicable to ruminants, because these latter
possess neither canine nor incisor teeth in the upper jaw (see p. 125,
dentition of the ox and sheep). The superior maxillary bone of one side
and that of the opposite side do not meet in the median line in the
region which corresponds to the incisor teeth; they are separated by a
bone which, in the human species, is present only at the commencement of
life, and afterwards coalesces with the maxilla; this is the
intermaxillary or incisor bone. This bone, which is paired, is formed of
a central part, which bears the superior incisor teeth; it is prolonged
upwards and backwards by two processes: one, external, which insinuates
itself between the superior maxillary and the nasal bone, except in the
sheep, in which it remains widely separated from the latter; the other,
internal, which is united to that which belongs to the bone of the
opposite side to form part of the floor of the cavity of the nasal
fossae; the external border of this process, which is separated from the
body of the bone by a notch, forms the internal boundary of the
corresponding _incisor opening_ or the _incisor slit_. Owing to the
absence of superior incisors in ruminants, the intermaxillary bone
presents no alveoli.
The malar bone, and the os unguis or lachrymal, are more or less
developed according to the species considered. With regard to the malar
bone, it is most important to notice the part which it takes in the
formation of the zygomatic arch, and that its inferior border
contributes to form the crest to which is attached the masseter muscle.
As for the nasal bones, they present differential characters which, as
they affect the form of the region which they occupy, are worthy of
notice.
[Illustration: FIG. 63.--SKULL OF THE HORSE: LEFT LATERAL ASPECT.
1, Posterior surface of the occipital bone; 2, external occipital
protuberance; 3, occipital condyle; 4, jugular process; 5, parietal
bone; 6, frontal bone; 7, orbital cavity; 8, zygomatic process of the
temporal bone; 9, external auditory canal; 10, mastoid process; 11,
superior maxillary bone or _maxilla_; 12, intermaxillary or incisor
bone; 13, nasal bone; 14, anterior orifice of the cavity of the nasal
fossae; 14', malar bone; 15, lachrymal bone; 16, inferior maxillary bone
or _mandible_; 17, inferior maxillary fissure; 18, condyle of the
inferior maxillary bone; 19, coronoid process of the inferior maxillary
bone; 20, incisor teeth; 21, canine teeth; 22, molar teeth.]
Their dimensions in length are proportional to those of the face. Very
small in man, they are more developed in carnivora. We recognise in the
latter the two curves which characterize them in the human species, and
which we clearly notice when we view them on one of their lateral
aspects: a concavity above, and a convexity below. These curves are more
or less accentuated--very strongly marked in the bulldog, and scarcely
at all in the greyhound. Moreover, in the carnivora also the nasal bones
are wider below than above, and form, by their junction, a semicircular
notch which limits, in its superior portion, the anterior opening of the
cavity of the nasal fossae. In the horse they present an opposite
arrangement with regard to their dimensions in width; broad above, each
terminates below by forming a pointed process which, separated from the
intermaxillary bones, is prolonged in front of the nasal orifice.
The inferior maxillary bone is, as in man, formed of a body and two
branches. But among the many special characteristics of form and size
which sharply differentiate it from the human bone, one detail must be
indicated; this is the absence of a mental prominence. Hence it results
that the anterior border of the body of the lower jaw, instead of being
directed obliquely downwards and forwards, is, on the contrary, oblique
downwards and backwards, and that in certain animals this border is
actually found almost exactly on the prolongation of the inferior border
of the body of the bone.
On the external surface of the body are found the three mental foramina.
The superior border is hollowed out by alveoli.
With regard to the branches (_rami_), they terminate in two processes:
one, the posterior, is the condyle; the other, situated more forwards,
is the coronoid process, which gives insertion to the temporal muscle.
These two processes are separated by the sigmoid notch.
For reasons which we will explain further on (see p. 127, movements of
the lower jaw), the condyle presents differences of form. In the
carnivora, it is strongly convex from before backwards, expanded
transversely, and firmly mortised in the glenoid cavity of the temporal
bone; in the ruminants, it is less convex from before backwards, it is
more slightly concave in the transverse direction; in the rodents--we
give as an example the hare (Fig. 64)--the condyle is still convex from
before backwards, but it is flattened from without inwards.
In the animals in which the muscles of mastication are very highly
developed, and especially in the carnivora, the osseous regions occupied
by these muscles are more extensive and more deep than in the human
species. The length of the coronoid process, the depth of the temporal
fossa, the extent of the zygomatic arch, the appearance of the external
surface of each of the rami of the lower jaw, deeply hollowed out for
accommodation of the masseter, and to provide extensive surfaces of
insertion for this muscle, are sure proofs furnished by the skeleton of
the occasionally enormous development of the muscles of mastication.
In the carnivora, a rather strong process, which is directed backwards,
occupies the angle of the inferior maxilla; it is, accordingly, situated
below the region of the condyle.
[Illustration: FIG. 64.--SKULL OF THE HARE: LEFT LATERAL ASPECT.
1, External occipital protuberance; 2, occipital condyle; 3, parietal
bone; 4, frontal bone; 5, orbital process; 6, orbital cavity; 7,
zygomatic process; 8, external auditory canal; 9, superior maxillary
bone; 10, intermaxillary or incisor bone; 11, nasal bone; 12, anterior
opening of the nasal fossa; 13, malar bone; 14, inferior maxillary bone;
15, condyle of the inferior maxillary bone; 16, incisor teeth; 17, molar
teeth.]
The teeth which the jaws carry vary in number, and even in appearance,
according to species; it is useful to note their differences. In order
to establish the nature of these latter more effectively, we will first
recall the fact that in man the teeth, thirty-two in number, are equally
distributed between the jaws, and are divided into incisors, canines,
and molars, of which the arrangement is thus formulated:
5_m._ 1_c._ 2_i._ | 2_i._ 1_c._ 5_m._
--------------------+-------------------- = 32.[19]
5_m._ 1_c._ 2_i._ | 2_i._ 1_c._ 5_m._
[19] _I.e._, _i_, incisors; _c_, canines; _m_, molars.
We also note that the incisors are edged, the canines are pointed, and
that the molars, cubical in shape, have their surface of contact
provided with tubercles.
The teeth of the cat are thirty in number; they are thus arranged:
4_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 4_m._
--------------------+-------------------- = 30.
3_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 3_m._
Those of the dog number forty-two:
6_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 6_m._
--------------------+-------------------- = 42.
7_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 7_m._
In these animals, the incisors, such as are not damaged by use, are
furnished, on the free border of their crown, with three tubercles, of
which one, the median, is more developed than those which are situated
laterally. We denote these teeth, commencing with those nearest the
median line, by the names _central incisors_ or _nippers_,
_intermediate_ and _corner incisors_. The canines, or _fangs_, are long
and conical; they are curved backwards and outwards. The upper canines,
which are larger than those of the lower jaw, are separated from the
most external of the incisors (_corner_) by an interval in which the
canines of the lower jaw are received. The lower canines, on the other
hand, are in contact with the neighbouring incisors, and are each
separated from the first molar which succeeds them by a wider interval
than that which is situated between the corresponding teeth in the upper
jaw.
The molars differ essentially from the teeth of the same class in the
human species. Their crown terminates in a cutting border bristling with
sharp-pointed projections; this formation indicates that these teeth are
principally designed for tearing. During the movement of raising the
lower jaw, which is so energetic in the carnivora, they act, indeed, in
the same manner as the two blades of a pair of scissors. The largest
molars are: in the dog, the fourth of the upper jaw, and the fifth in
the opposite one; in the cat, the third both above and below.
The pig has forty-four teeth disposed in the following manner:
7_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 7_m._
--------------------+-------------------- = 44.
7_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 7_m._
Of the incisors, the nippers and the intermediate ones of the upper jaw
have their analogues in those of the horse; in the lower jaw, the
corresponding teeth, straight, and directed forward, rather resemble the
same incisors in rodents. The corner incisor teeth are much smaller, and
are separated from the neighbouring teeth. The canine teeth, also called
_tusks_ or _tushes_, are greatly developed, especially in the male. The
molars increase in size from the first to the last; they are not
cutting, as in the carnivora, but they are not flattened and provided
with tubercles on their surfaces of contact as in the herbivora.
In the ox and the sheep the teeth are thirty-two in number:
6_m._ 0_c._ 0_i._ | 0_i._ 0_c._ 6_m._
--------------------+-------------------- = 32.
6_m._ 0_c._ 4_i._ | 4_i._ 0_c._ 6_m._
As we see from this dental formula, the incisors are found only in the
lower jaw; they are replaced in the upper jaw by a thick cartilaginous
pad on which the inferior incisors find a surface of resistance.
These have their crowns flattened from above downwards, and gradually
become thinner from the root to the anterior border, which is edged and
slightly convex. These teeth gradually wear away. In proportion to the
progress of this wear, on account of the fact that it involves the
anterior borders and upper surfaces of the incisor teeth, and that these
teeth are narrower towards the root than at the opposite extremity, the
intervals which separate them tend to become wider and wider; and when
the roots become exposed by the retraction of the gums, they are then
separated from one another by a considerable interval. The molars have
their grinding surface comparable to that of the horse; they increase in
size from the first to the sixth.
The teeth of the horse are forty in number; they are thus distributed:
6_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 6_m._
--------------------+-------------------- = 40.
6_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 6_m._
As they become worn, these teeth continue to grow, and as, on the one
hand, this phenomenon takes place throughout the whole life of the
animal, and, on the other hand, the process of wear brings out and makes
visible at the surface of friction parts formerly deeper and deeper, and
of which the configuration varies at different levels, there result
special features which permit the determination of the age of the animal
by an examination of its jaws. The incisors are called, commencing with
those situated nearest the middle line, _central incisors_ or _nippers_,
_intermediate_ and _corner incisors_. The canines, also designated as
the _fangs_, exist only in the male. It is exceptional to find them in
the mare, and when they exist in this latter they are less developed
than those of the horse. The molars have cuboid crowns; the surface of
friction is almost square in the case of the upper molars, and is
inclined so as to look inwards; in the case of the inferior ones, it is
a little narrowed, and is inclined so as to look outwards. In the upper
jaw the external surface of the crown is hollowed by two longitudinal
furrows; in the lower jaw the same surface has only one furrow, which at
times is but slightly marked.
In the hare the teeth are twenty-eight in number:
6_m._ 0_c._ 2_i._ | 2_i._ 0_c._ 6_m._
--------------------+-------------------- = 28.
5_m._ 0_c._ 1_i._ | 1_i._ 0_c._ 5_m._
The four incisors of the upper jaw are divided into two groups; one of
these is formed by the two principal teeth, the other by two very small
incisors which are placed behind the preceding.
Having studied the jaws and examined the arrangement of the teeth, we
should say a few words on the movements which the lower jaw is able to
execute. In man, these movements are varied in character: the jaw is
lowered and raised; it can also be projected forwards and drawn
backwards, or carried to the right or left side by lateral movements.
Owing to the different modes of nutrition of animals, with which the
shape of the teeth is clearly correlated, being more specialized than in
the human species, the lower jaw is moved in a fashion less varied and
in the direction most suitable for the mastication of the foods which
form the aliment of the species considered. Moreover, this is plainly
shown in the skeleton by the shape of the condyle of the lower jaw (see
p. 122, different forms of this condyle). In the carnivora, whose teeth,
as we have seen, are all cutting ones, the jaw rises and falls; the food
then is, if we consider the two jaws, cut as by the blades of a pair of
scissors. In the ruminants, the incisors exist only in the lower jaw,
but the molars are thick and well developed; the food is ground by these
latter as by millstones, and the movements which favour this action are,
above all, the lateral. As for the rodents, in which the incisors are
formed for filing down and cutting through hard resisting bodies, their
lower jaw moves in the antero-posterior direction, in such a way that
the inferior incisors alternately advance and recede beneath those of
the upper jaw. The free cutting border of these teeth effectively
fulfils the function to which they are destined; their constant wear
preserves and revivifies the chisel edge which characterizes them,
without leading to their destruction, for the incisors in rodents are of
continuous growth.
THE SKULL OF BIRDS
=The Skull of Birds= (Fig. 65).--If, because it is less important from
the artistic point of view, we do not consider it necessary to describe
in detail the skull of birds, we yet think it useful to indicate, in
their general lines, the peculiarities it presents.
In this group the skull is generally pear-shaped; to the cranium, of
which the bones are arranged in such a way as to give it a form more or
less spherical, succeeds a face more or less elongated, according as the
bill is more or less developed.
In general, the bones of the skull coalesce very early, with the result
that it is only in very young individuals that we can determine their
presence.
We find the skull to consist of an occipital bone, two parietals, a
frontal, etc.; we will indicate but one detail in connection with these
bones: it is the presence of a single condyle for the articulation of
the occipital bone with the atlas. We also note the quadrate bone, which
is situated on the lateral part of the cranium, is movable on this
latter, and acts as an intermediary between it, the bones of the face,
and the lower jaw. The quadrate bone is regarded as a detached portion
of the temporal; on the signification of this we do not now propose to
dwell.
[Illustration: FIG. 65.--SKULL OF THE COCK: LEFT LATERAL SURFACE.
1, Occipital bone; 2, parietal bone; 3, frontal bone; 4, ethmoid bone;
5, cavity of the tympanum; 6, quadrate bone; 7, superior maxillary bone;
8, malar bone; 9, nasal bone; 10, 10, intermaxillary bone; 11, nasal
orifice; 12, os unguis or lachrymal bone; 13, inferior maxillary bone.]
On the anterior portion of the face we find the nasal bones, which,
articulating with the frontal on one side, circumscribe, on the other,
the posterior border of the nares. The nasal bone of the one side is
separated from that of the opposite by the intermaxillary or
premaxillary bone, which forms the skeleton of the superior mandible.
The superior maxillaries, which are rudimentary, are situated on the
lateral parts, and prolonged backward by an osseous style which
articulates with the quadrate bone; this styloid bone, the homologue of
the malar, is designated by certain authors as the _jugal_ or
_quadrato-jugal_ bone.
It is with the quadrate bone also that the inferior maxillary
articulates.