Part 12

FIG. 64.—RIGHT FEMUR OF HORSE, POSTERIOR VIEW. ]

The =proximal extremity= (Extremitas proximalis) is large and consists of the head, neck, and great trochanter. The =head= (Caput femoris) is placed at the inner side and is directed inward, upward, and somewhat forward. It is approximately hemispherical and articulates with the acetabulum. It is cut into internally by a deep notch, the =fovea capitis=, in which the pubo-femoral and round ligaments are attached. The articular surface is surrounded by a distinct margin. The =neck= (Collum femoris) is most distinct in front and internally. The =great trochanter= (Trochanter major) is situated externally; it presents three features. The =anterior part= or =convexity= is situated opposite to the head and rises little above the level of the latter; it gives attachment to the deep gluteus muscle, and in the fresh state its outer surface is coated with cartilage, over which a tendon of the middle gluteus passes, to be inserted into the =crest=, which is placed below and behind the convexity. The =posterior part= or =summit= is separated from the convexity by a notch; it is situated behind the plane of the head and rises to a much greater height. It furnishes insertion to part of the middle gluteus muscle. Its posterior border is continued downward as the =trochanteric ridge=, which forms the outer wall of the =trochanteric fossa=. A number of foramina are found in the concave area internal to the convexity.

The =distal extremity= (Extremitas distalis) is large in both directions and comprises the trochlea in front and two condyles behind. The =trochlea= consists of two ridges separated by a groove, and forms an extensive surface (Facies patellaris) for articulation with the patella. It is very unsymmetrical; the inner ridge or lip is much wider, more prominent, and extends up higher than the external one, and the two converge below. The =condyles=, =internal= and =external= (Condylus medialis, lateralis), are separated by the deep =intercondyloid fossa= (Fossa intercondyloidea), and articulate with the condyles of the tibia and the semilunar cartilages of the stifle joint. A ridge connects each condyle with the lower part of the corresponding lip of the trochlea. The intercondyloid fossa lodges the spine of the tibia and the crucial ligaments of the stifle joint, which are attached here.

The condyles are obliquely placed, with their long axes directed downward, forward, and inward. The articular surface of the external condyle is more strongly convex from side to side than that of the inner one, and the ridge which connects it with the trochlea is much narrower.

The =internal epicondyle= (Epicondylus medialis) is a rounded prominence on the internal surface of the internal condyle, to which the internal lateral ligament and the adductor muscle are attached. The corresponding =external epicondyle= (Epicondylus lateralis) is less distinct; it presents a mark where the lateral ligament is attached, below and behind which there is a depression (Fossa musculi poplitei) in which the popliteus muscle arises. Between the external condyle and trochlea is the =extensor fossa= (Fossa extensoria), in which the tendon of origin of the anterior extensor and peroneus tertius is attached.

=Development.=—The shaft and the distal end each ossify from one center, but the proximal end has two centers, one of which is for the great trochanter. The edge of the external trochanter also has a separate center. The proximal end fuses with the shaft at three to three and a half years, the distal at about three and a half years.

THE TIBIA

The =tibia= is a long bone which extends obliquely downward and backward from the stifle to the hock. It articulates above with the femur, below with the tarsus, and externally with the fibula. It possesses a shaft and two extremities.

The =shaft= or =body= (Corpus tibiæ), large and three-sided above, becomes smaller and flattened in the sagittal direction below, but widens a little at the distal end. It presents for notice three surfaces and three borders. The =internal surface= (Facies medialis) is broad above, where it furnishes insertion to the internal lateral ligament and the sartorius and gracilis muscles; below this it is convex and subcutaneous. The =external surface= (Facies lateralis) is smooth and somewhat spiral. It is wide and concave in its upper part, below which it becomes narrower and slightly convex, and winds gradually to the front of the bone; near the distal end it widens a little, becomes flat, and faces forward. The =posterior surface= (Facies posterior) is flattened, and is divided into two parts by the rough =popliteal line=, which runs obliquely from the upper part of the external border to the middle of the internal border. The triangular area above the line is occupied by the popliteus muscle, while the area below is marked by rough lines (Lineæ musculares) to which the deep flexor muscle of the digit is attached; the lines fade out below, where the surface is smooth and flat. The =nutrient foramen= is situated on or near the popliteal line. The =anterior border= is very prominent in its upper third, forming the =tibial crest= (Crista tibiæ); below it is reduced to a rough line, which ends at a small elevation near the distal end of the bone. The =internal border= (Margo medialis) is rounded in its upper half, to which the popliteus muscle is attached, and a tubercle is found on this part. The lower part is a rough line on well-marked bones. The =external border= (Crista interossea) is concave in its upper part and concurs with the fibula in the formation of the interosseous space of the leg; a smooth impression indicates the course of the anterior tibial vessels through the space to the front of the leg. Lower down the border divides and incloses a narrow triangular surface.

The =proximal extremity= (Extremitas proximalis) is large and three-sided. It bears two lateral eminences, the =internal= and =external condyles= (Condylus medialis, lateralis). Each presents a somewhat saddle-shaped surface for articulation with the condyle of the femur and the semilunar cartilage. The =spine= or =intercondyloid eminence= (Eminentia intercondyloidea) is the central prominence, upon which the articular surfaces are continued; it consists of a high inner part and a lower outer part (Tuberculum intercondyloideum mediale, laterale). On, before, and behind the spine are the =anterior= and =posterior intercondyloid fossæ=, in which the anterior crucial ligament and the semilunar cartilages are attached. The condyles are separated behind by the deep =popliteal notch= (Incisura poplitea), on the inner side of which is a tubercle for the attachment of the posterior crucial ligament. The external condyle has an overhanging outer margin, below which is a =facet= for articulation with the fibula. The =large anterior= eminence is the =tuberosity= of the tibia (Tuberositas tibiæ). It is marked in front by a =groove=, the lower part of which gives attachment to the middle patellar ligament, and the groove is flanked by rough areas for the attachment of the internal and external patellar ligaments. A semicircular smooth =notch= (Sulcus muscularis) separates the tuberosity from the external condyle, and gives passage to the tendon of origin of the anterior extensor and the peroneus tertius.

The =distal extremity= (Extremitas distalis) is much smaller than the proximal; it is quadrangular in form and larger internally than externally. It presents an =articular surface= (Cochlea tibiæ), which is adapted to the trochlea of the tibial tarsal bone (astragalus), and consists of two grooves separated by a ridge. The ridge and grooves are directed obliquely forward and outward, and are bounded laterally by the =malleoli=, to which the lateral ligaments of the hock joint are attached. The =internal malleolus= (Malleolus tibialis) is the more prominent of the two, and forms the anterior boundary of a groove for the tendon of the inner head of the flexor perforans. The =external malleolus= (Malleolus fibularis) is broader, and is marked by a vertical groove for the passage of the lateral extensor tendon.

=Development.=—The tibia has the usual three chief centers of ossification and supplementary ones for the tuberosity and the external malleolus. The latter is really the distal end of the fibula; it is a separate piece at birth, and the line of union is commonly quite evident in the adult in the external articular groove. The proximal end unites with the shaft at about three and a half years, and the distal end at about two years of age.

THE FIBULA

The =fibula= of the horse is a much reduced long bone, situated along the outer side of the tibia.

The =shaft= or =body= (Corpus fibulæ) is a slender rod which forms the outer boundary of the interosseous space; it usually terminates below in a pointed end about one-half to two-thirds of the way down the external border of the tibia.

The =proximal extremity= or =head= (Capitulum fibulæ) is relatively large, and is flattened transversely. Its internal surface presents a narrow area along the upper border for articulation with the tibia. The external surface is rough and gives attachment to the external lateral ligament of the stifle joint. It has rounded anterior and posterior borders.

The =distal extremity= is fused with the tibia, constituting the external malleolus.

=Development.=—This resembles that of the ulna. The embryonic cartilaginous fibula extends the entire length of the leg, but does not articulate with the femur. The lower part of the shaft is usually reduced to a fibrous band. Three centers of ossification appear, one each for the shaft and the extremities. The distal end unites early with the tibia, forming the external malleolus.

It is interesting to note that in some cases the entire shaft of the fibula develops, a reversion to the condition in the Miocene ancestors of the present horse.

THE PATELLA

The =patella= is a large sesamoid bone which articulates with the trochlea of the femur. It presents for description two surfaces, two borders, a base, and an apex.

The =anterior= or =free surface= (Facies libera) is irregularly quadrilateral, convex, and rough for muscular and ligamentous attachment.

The =posterior= or =articular surface= is smaller and is triangular in outline. It presents a vertical rounded ridge, which corresponds to the groove on the trochlea of the femur, and separates two concave areas. Of the latter, the inner cavity is much the larger, and is not very well adapted to the inner lip of the trochlea; in the fresh state, however, it is completed and rendered more congruent by the curved =accessory fibro-cartilage=.

The =lateral borders= converge to the apex below, and each forms a prominence or angle at the base. The inner angle and the adjacent part of the border give attachment to the fibro-cartilage of the patella, which increases the articular surface. The external border is rounded and its angle is less prominent.

The =base= (Basis patellæ) faces upward and backward, and is convex transversely, concave from before backward.

The =apex= forms a blunt point directed downward.

=Development.=—The patella develops as a sesamoid bone from a single center in a cartilaginous deposit in the tendon of the quadriceps femoris muscle.

THE TARSUS

The =tarsus= or =hock= of the horse usually comprises six short bones (Ossa tarsi), but exceptionally seven are present.

THE TIBIAL TARSAL BONE

The =tibial tarsal= bone (Os tarsi tibiale, astragalus, or talus) is the inner bone of the proximal row. It is extremely irregular in form, but may be considered as offering six surfaces for description.

The =superior= and the =anterior= or =dorsal surface= are continuous, and form a =trochlea= for articulation with the distal end of the tibia. The trochlea consists of two oblique ridges with a deep groove between them, which curve spirally forward, downward, and outward. There is usually a shallow =synovial fossa= in the groove. The =inferior surface= is convex from before backward, and most of it articulates with the central tarsal; externally it has an oblique facet for the fourth tarsal, and a non-articular groove cuts into the surface to its middle. The =posterior= or =plantar surface= is oblique and extremely irregular; it presents four facets for articulation with the fibular tarsal bone; the facets are separated by rough excavated areas, and the largest fossa (Sulcus tali) forms with a corresponding one on the fibular tarsal a cavity termed the =sinus tarsi=. The =internal surface= bears on its lower part a large tuberosity, and on its upper part a small one for the attachment of the internal lateral ligament. The =external surface= is smaller than the internal, and is marked by a wide rough fossa in which the external ligament is attached.

THE FIBULAR TARSAL BONE

The =fibular tarsal= bone (Os tarsi fibulare, calcaneum, os calcis) is the largest bone of the hock. It is elongated, flattened from side to side, and forms a lever for the muscles which extend the hock joint. It consists of a body and an inner process, the sustentaculum tali.

The =body= (Corpus calcanei) is enlarged at its proximal end to form the =tuber calcis= or “point of the hock.” The posterior part of this eminence gives attachment to the tendon of the gastrocnemius, while in front and laterally it furnishes insertion to tendons of the flexor perforatus, biceps, and semitendinosus muscles. The =inferior extremity= bears a concave facet for articulation with the fourth tarsal bone. The =internal surface= has on its lower part a strong process, the =sustentaculum tali=, which projects inward. The process has a large, oval, slightly concave facet in front for articulation with the tibial tarsal, and sometimes a small articular surface below for the central tarsal bone. Its plantar surface forms with the smooth inner surface of the body a groove for the deep flexor tendon (Sulcus musculi flexoris hallucis longi). Its inner surface has a prominence on the lower part for the attachment of the lateral ligament. The =external surface= of the body is flattened, except below, where there is a rough prominence for the attachment of the lateral ligament. The =anterior= or =dorsal border= is concave in its length, smooth and rounded in its upper part. About its middle is a blunt-pointed projection (Processus cochlearis) which bears facets on its inner and lower surfaces for articulation with the tibial tarsal bone, and is roughened outwardly for ligamentous attachment. Below this are two facets for the tibial tarsal, and an extensive rough fossa which concurs in the formation of the sinus tarsi. The =posterior= or =plantar border= is straight and thick, and widens a little at either end; it is rough, and gives attachment to the long plantar ligament.

THE CENTRAL TARSAL BONE

The =central tarsal= bone (Os tarsi centrale, scaphoid, or navicular) is irregularly quadrilateral, and is situated between the tibial tarsal above and the third tarsal below. It is flattened from above downward, and may be described as having two surfaces and four borders. The =superior surface= is concave from before backward, and almost all of it articulates with the tibial tarsal; a non-articular depression cuts into its outer part, and sometimes there is a facet for the fibular tarsal bone on the posterior angle. The =inferior surface= is convex, and is crossed by a non-articular groove, which separates facets for articulation with the third and the first and second (fused) tarsals. The =anterior= or =dorsal border= and the =internal border= are continuous, convex, and rough. The =posterior= or =plantar border= bears two prominences, separated by a notch. The =external border= is oblique, and bears anterior and posterior facets for articulation with the fourth tarsal, between which it is excavated and rough.

FIRST AND SECOND TARSAL BONES

The =first= and =second tarsal= bones (Os tarsale primum et secundum, cuneiform parvum) are usually fused in the horse, forming a bone of very irregular shape, situated in the inner and posterior part of the lower row, below the central and behind the third tarsal. It is the smallest of the tarsal bones, and may be described as having two surfaces, two borders, and two extremities. The =internal surface= faces backward and inward, and is convex. Its anterior part is ridged, and gives attachment to the internal lateral ligament, and its posterior part bears an imprint where the inner tendon of the tibialis anterior is inserted. The =external surface= is marked by a deep notch which indicates the division between the first and second tarsal elements; it bears on its anterior part a large concave facet for the central tarsal. The =superior border= is convex. The =inferior border= is broad in front, where it articulates with the large and inner small metatarsal bones. The =anterior extremity= has a small facet for articulation with the third tarsal, and bears internally a ridge or tubercle. The =posterior extremity= is a blunt point.

In some cases the first and second tarsal bones remain separate—a remarkable reversion to the condition in the early ancestors of the horse. In such specimens the first tarsal is a discoid bone, articulating above with the central, below with the small metacarpal bone. The second tarsal is quadrangular, equivalent to the thick anterior part of the bone as described above, and overlapped by the anterior part of the first tarsal.

THE THIRD TARSAL BONE

The =third tarsal= bone (Os tarsale tertium, third or great cuneiform) resembles the central, but is smaller and triangular in outline. It is situated between the central above and the large metatarsal bone below. It possesses two surfaces and three borders.

The =superior surface= is concave, and is crossed by a non-articular depression which divides it into two unequal facets; it articulates with the central tarsal. The =inferior surface= is slightly convex, and rests on the large metatarsal bone; it has an extensive central rough excavation. The =anterior= or =dorsal border= is convex and bears a rounded ridge on its inner part. The =internal border= is deeply notched and has a small facet for the second tarsal on its anterior part. The =external border= is also divided by a notch into two parts, and bears two diagonally opposite facets for articulation with the fourth tarsal. In some cases there is a facet for the inner small metatarsal bone.

THE FOURTH TARSAL BONE

The =fourth tarsal= bone (Os tarsale quartum, cuboid) is the outer bone of the lower row, and is equal in height to the central and third together. It is cuboid in shape and presents six surfaces.

The =superior surface= is convex from side to side, and articulates chiefly with the fibular tarsal, but to a small extent with the tibial tarsal also. The =inferior surface= rests on the large and external small metatarsal bones. The =internal surface= bears four facets for articulation with the central and third tarsal bones. It is crossed from before backward by a smooth groove, which by apposition with the adjacent bones forms the =canal of the tarsus= (Canalis tarsi) for the passage of the perforating tarsal vessels. The =anterior= or =dorsal=, =external=, and =posterior= or =plantar surfaces= are continuous and rough. A tuberosity behind gives attachment to the plantar ligament.

=Development.=—The fibular tarsal bone has two centers of ossification, one for the main mass and the other for the tuber calcis; the latter fuses with the rest of the bone at about three years of age. The first and second tarsals have separate centers, but fusion usually occurs before birth. Each of the other bones ossifies from a single center.

THE METATARSUS

The =metatarsal= bones (Ossa metatarsalia), three in number, have the same general arrangement as the metacarpal bones, but present some important differences. Their direction is slightly oblique, downward and a little forward.