[Ill.u.s.tration: 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, princ.i.p.al 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 h.o.m.ologue 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 princ.i.p.al metatarsal; the external half of this metatarsal articulates with the portion of bone which represents the cuboid.

The metatarsus is represented by a princ.i.p.al 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 princ.i.p.al 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 b.u.t.tock_.

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 pa.s.s 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_.

[Ill.u.s.tration: 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 pa.s.sage 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 p.r.o.nounced 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.

[Ill.u.s.tration: FIG. 54.--TARSUS OF THE HORSE: LEFT POSTERIOR LIMB, ANTERIOR SURFACE.

1, Tibia; 2, internal tuberosity of the inferior extremity of the tibia (h.o.m.ologue of the internal malleolus of man); 3, external tuberosity of the inferior extremity of the tibia (h.o.m.ologue 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, princ.i.p.al 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 a.n.a.logous to that of these latter. Accordingly, we shall merely indicate the differences which characterize them.

The princ.i.p.al metatarsal is longer than the metacarpal of the same cla.s.s; 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 c.o.xo-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 circ.u.mference, and is attached to the anterior border of the p.u.b.es, 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.