Chapter VII: Introduction (5)

The =pelvic surface= (Facies pelvina) faces in the opposite direction; it is convex, and consists of two distinct parts. The inner triangular part (Pars articularis) is roughened for ligamentous attachment, and bears an irregular facet, the =auricular surface= (Facies auricularis), for articulation with the sacrum. The outer quadrilateral part (Pars iliaca) is in general smooth. It is crossed by the =ilio-pectineal line= (Crista iliopectinea), which begins below the auricular surface and is continued on the shaft of the bone to join the anterior border of the pubis. The line is interrupted by furrows for the iliaco-femoral vessels, and below these it bears the =psoas tubercle= (Tuberculum psoadicum), which gives attachment to the psoas minor muscle. The iliacus muscle is attached to the surface external to the ilio-pectineal line.

The =anterior border= or =crest= (Crista iliaca) is concave, thick, and rough.

The =internal border= is deeply concave; its middle part forms the lower boundary of the =great sciatic foramen=, and it is continuous behind with the =superior ischiatic spine=.

The =external border= is concave and in great part rough. Its fore part is crossed by grooves for the ilio-lumbar vessels, which are continued on the pelvic surface. The =nutrient foramen= is usually situated on or near the posterior part of this border.

FIG. 61.—OSSA COXARUM OF MARE, VENTRAL VIEW.

_D_, Ilium; _Sch._, pubis; _S_, ischium; _a_, anterior border (crest)
of ilium; _b_, internal angle of ilium; _c′_, external angle of
ilium; _d_, great sciatic notch; _e_, external border of ilium; _f_,
iliac surface; _g_, linea arcuata; _h_, rough ligamentous area; _i_,
auricular surface; _k_, ilio-pectineal line; _l_, psoas tubercle;
_m_, transverse branch, and _n_, symphyseal branch of pubis; _o_,
_o′_, symphysis pelvis; _p_, anterior border of pubis (pecten); _q_,
tuberculum pubicum; _r_, ilio-pectineal eminence; _s_, tuber ischii;
_t_, ischial arch; _u_, lesser sciatic notch; _v_, acetabular
branch, and _w_, symphyseal branch of ischium; _x_, obturator
foramen; _y_, articular surface of acetabulum; _z_, fossa acetabuli;
_1_, groove for ilio-lumbar artery; _2_, groove for iliaco-femoral
artery; _3_, subpubic groove; _4_, depression for inner tendon of
origin of rectus femoris; _5_, rough area for attachment of adductor
muscles. (Ellenberger-Baum, Anat. d. Haustiere.)
]

The =internal= or =sacral angle= (Tuber sacrale) curves upward and a little backward close to the first sacral spine, and forms here the highest point of the skeleton. It is somewhat thickened and rough.

The =external= or =coxal angle= (Tuber coxæ) forms the basis of the point of the hip. It is a large quadrangular mass, narrow in its middle, and enlarged at either end, where it bears a pair of tuberosities. It is roughened for muscular attachment.

The =posterior= or =acetabular angle= meets the other two bones at the acetabulum, of which it forms about two-fifths. Its prominent upper border forms part of the =superior ischiatic spine=, which is roughened externally, smooth internally. Two depressions above and in front of the acetabulum give attachment to the tendons of origin of the rectus femoris muscle. This angle is connected with the ala or wide part of the bone by a constricted part, often termed the =shaft=. The latter is of three-sided prismatic form. Its external surface is convex and rough, and gives attachment to the deep gluteus muscle. Its pelvic surface is smooth and is grooved for the obturator vessels and nerves. Its ventral surface is crossed by vascular grooves, below which is a rough area, bounded internally by the psoas tubercle.

THE ISCHIUM

The =ischium= (Os ischii) forms the posterior part of the ventral wall or floor of the bony pelvis. It slopes a little downward and inward, but is practically horizontal in the longitudinal direction. It is irregularly quadrilateral, and may be described as having two surfaces, four borders, and four angles.

The =superior= or =pelvic surface= (Facies pelvina) is smooth and slightly concave from side to side.

The =inferior surface= (Facies externa) is nearly flat, and is in great part roughened for the attachment of the adductor muscles.

The =anterior border= forms the posterior margin of the obturator foramen.

The =posterior border= is thick and rough. It slopes obliquely inward and forward to meet the border of the other side, forming with it the =ischial arch= (Arcus ischiadicus).

The =internal border= meets the opposite bone at the symphysis.

The =external border= is thick and rounded, but concave in =its length=; it forms the =lesser sciatic notch=, the lower boundary of the lesser sciatic foramen.

The =antero-internal angle= or =symphyseal branch= (Ramus symphyseos) meets the pubis, with which it forms the inner boundary of the obturator foramen.

The =antero-external angle= or =acetabular branch= joins the other two bones at the acetabulum, of which it forms more than half. Superiorly it bears part of the =superior ischiatic spine= (Spina ischiadica), and internally it is grooved for the obturator vessels.

The =postero-external angle= is a thick three-sided mass, the =tuber ischii= (Tuber ischiadicum); its lower border is the =inferior ischiatic spine=, to which the biceps femoris and semitendinosus muscles are attached.

THE PUBIS

The =pubis= (Os pubis) is the smallest of the three parts of the os coxæ. It forms the anterior part of the pelvic floor, and may be described as having two surfaces, three borders, and three angles.

The =superior= or =pelvic surface= (Facies pelvina) is convex in the young subject and the stallion, concave and smooth in the mare and usually in the gelding also. The urinary bladder rests on it.

The =inferior= or =ventral surface= (Facies externa) is convex, and in great part rough for muscular attachment. Near the anterior border it is crossed by the =subpubic groove=, the inner part of which is occupied by a large vein, the outer part by the pubo-femoral ligament.

The =anterior border= is thin in its inner part (except in the young subject and the stallion), forming the pecten ossis pubis. Externally it bears the rough =ilio-pectineal eminence= (Eminentia iliopectinea), beyond which it is continuous with the ilio-pectineal line.

The =internal border= joins the opposite bone at the symphysis pubis.

The =posterior border= forms the anterior margin of the obturator foramen, and is marked externally by the obturator groove.

The =internal angle= meets its fellow at the anterior end of the symphysis. This part is very thick in the young subject and the stallion, but in the mare, and usually in the gelding also, it becomes thin with advancing age.

The =external= or =acetabular angle= joins the ilium and ischium at the acetabulum.

The =posterior angle= joins the ischium, with which it forms the inner boundary of the obturator foramen.

The =pubis= may conveniently be regarded as consisting of two branches which meet at a right angle; these are termed the =transverse= or =acetabular branch= (Ramus acetabularis) and the =longitudinal= or =symphyseal branch= (Ramus symphysecs).

The =acetabulum= is a cotyloid cavity which lodges the head of the femur. It faces downward and outward, and consists of an articular and a non-articular part. The =articular part= (Facies lunata) is crescentic, and is cut into internally by the non-articular part, which lies at a lower level, and is termed the =acetabular fossa= (Fossa acetabuli). The inner part of the rim is correspondingly cut into by the =acetabular notch= (Incisura acetabuli), which is converted into a foramen by the transverse ligament in the fresh state, and transmits the pubo-femoral and round ligaments to the head of the femur.

The =obturator foramen= (Foramen obturatum) is situated between the pubis and ischium. It is oval in outline, the longer axis being directed forward and outward. Its margin is grooved antero-externally for the obturator nerve and vessels.

FIG. 62.—OSSA COXARUM OF STALLION, DORSAL VIEW. (After
Ellenberger-Baum, Anat. für Künstler.)
]

=Development.=—Each division of the os coxæ ossifies from one chief center. The center for the ilium appears first near the acetabulum, followed quickly by one for the ischium, and a little later by the pubic center. Secondary centers appear for the crest and external angle of the ilium, the tuber and posterior border of the ischium, and the acetabular part of the pubis. The pubis and ischium are united at birth or soon after, but are not fused with the ilium until the second year. The epiphyseal parts fuse with the main mass at four and a half to five years of age.

The acetabular part of the pubis ossifies from a separate center. It
is most distinct in the embryo at three months, and is often called
the os acetabuli. Martin says that the ilium has a center for the
acetabular part, one for the shaft and wing, and a third for the
crest. He also states that there is a special center for the
acetabular part of the ischium, and a transitory nucleus in the
symphyseal part of the pubis.

THE PELVIS

The =bony pelvis= is composed of the ossa coxarum, the sacrum, and the first three coccygeal vertebræ. The =dorsal wall= or =roof= is formed by the sacrum and first three coccygeal vertebræ, and the =ventral wall= or =floor= by the pubic and ischial bones. The =lateral walls= are formed by the ilia and the acetabular part of the ischia. The defect in the skeleton here is supplied in the fresh state by the sacro-sciatic ligaments and semimembranosus muscles.

The =anterior aperture= or =inlet= (Apertura pelvis cranialis) is bounded by the =terminal line= (Linea terminalis) or brim, composed of the base of the sacrum dorsally, the ilio-pectineal lines laterally, and the anterior border of the pubis ventrally. It is almost circular in the mare, semi-elliptical in the stallion, and faces obliquely downward and forward. It has two principal diameters. Of these, the =conjugate= or =sacro-pubic diameter= (Conjugata) is measured from the sacral promontory to the anterior end of the symphysis. The =transverse diameter= (Diameter transversa) is measured at the greatest width, _i. e._, just above the psoas tubercle.

The =posterior aperture= or =outlet= (Apertura pelvis caudalis) is much smaller and is very incomplete in the skeleton. It is bounded above by the third coccygeal vertebra and below by the ischial arch; in the fresh state it is completed laterally by the sacro-sciatic ligament and the semimembranosus muscle.

The =axis= of the pelvis is an imaginary line drawn through the centers of the inlet, cavity, and outlet.

=Sexual Differences.=—Marked differences exist in the size and form of the pelvis in the two sexes. The average =conjugate diameter= is about 9½ inches (ca. 23 to 24 cm.) in the mare, 7½ inches (ca. 18 to 20 cm.) in the stallion. The =transverse diameter= of the inlet averages about 9 inches (ca. 22 to 23 cm.) in the mare, and 8 inches (ca. 20 cm.) in the stallion. The =obliquity= of the inlet is greater in the female; the difference is indicated by the fact that a vertical plane from the pecten cuts the fourth sacral segment in the female, the second in the male. The =outlet= is also larger in the mare, the =ischial arch= being about one-third wider than in the stallion. The =cavity= is much more roomy in the female; the transverse diameter between the middles of the superior ischiatic spines is about 8 inches (20 cm.) in the mare, 6 inches (15 cm.) in the stallion. The pubic part of the =floor= in the female is concave and lies considerably lower than the ischiatic part, which is wide and relatively flat. In the stallion the pubis is very thick centrally, and this part of the floor is convex, while the ischial part is relatively narrow, and is concave from side to side. The =obturator foramina= are correspondingly larger in the female. The ilium is shorter, and the greater sciatic notch deeper and narrower in the male. The pelvis of the gelding, when castration has been performed early, resembles that of the mare; otherwise the male characters appear to be retained to a large degree.

THE FEMUR

The =femur= or thigh bone (Os femoris) is the largest and most massive of the long bones. It extends obliquely downward and forward, articulating with the acetabulum above and the tibia and patella below. It presents for examination a shaft and two extremities.

The =shaft= or =body= (Corpus femoris) is in general cylindrical, but flattened behind, and larger above than below. The =anterior= and =lateral surfaces= are continuous and strongly convex from side to side; there is often a central vertical rough line on the proximal part, but otherwise these surfaces are smooth. They are covered by the quadriceps femoris muscle. The =posterior surface= is wide, flat, and smooth in its proximal fourth. Below this part there is a rough elevation externally for the attachment of the femoral tendon of the biceps femoris, and a rough line internally to which the quadratus femoris is attached. The middle third is narrower, and is rough for the attachment of the adductor muscle. Just below this area an oblique groove crosses the surface, indicating the position of the femoral vessels. The =internal border= bears on its proximal part the =internal trochanter= or =trochanter minor=, a thick rough ridge, to which the ilio-psoas muscle is attached. From this a rough line curves up to the front of the neck and indicates the limit of the attachment of the vastus internus muscle. A narrow rough area about the middle gives attachment to the pectineus muscle, and the =nutrient foramen= is usually found just in front of this mark. The =supracondyloid crest= is situated below the groove for the femoral vessels, and gives origin to the inner head of the gastrocnemius. The =external border= is prominent in its upper part, and bears at the junction of its proximal and middle thirds the =external= or =third trochanter= (Trochanter tertius); this process is curved forward, and furnishes insertion to the tendon of the superficial gluteus muscle. At the lower part is found the =supracondyloid= or =plantar fossa= (Fossa plantaris), in which the superficial flexor arises; it is bounded externally by a thick rough margin, to which the outer head of the gastrocnemius muscle is attached.

FIG. 63.—RIGHT FEMUR OF HORSE, EXTERNAL VIEW.
]

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

FIG. 65.—RIGHT FEMUR OF HORSE, INTERNAL VIEW.

_1_, Anterior part, _1′_, posterior part of trochanter major; _2_,
neck; _3_, fovea capitis; _4_, crest; _5_, trochanteric fossa; _6_,
external or third trochanter; _7_, trochanter minor; _8_, eminence
for attachment of biceps femoris; _9_, internal border; _10_,
nutrient foramen; _11_, groove for femoral vessels; _12_,
supracondyloid crest; _13_, supracondyloid fossa; _14_, trochlea;
_15_, external epicondyle; _16_, external condyle; _17_, extensor
fossa; _18_, internal condyle; _19_, internal epicondyle; _20_,
intercondyloid fossa. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

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.

FIG. 66.—FRONTAL SECTION OF LEFT FEMUR OF HORSE, ANTERIOR VIEW.

The figure shows that the medullary cavity is traversed for the most
part by fine bony trabeculæ.
]

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.

FIG. 67.—DISTAL EXTREMITY OF RIGHT FEMUR OF HORSE, END VIEW.
]

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.

FIG. 68.—RIGHT TIBIA AND FIBULA OF HORSE, EXTERNAL VIEW.
]

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.

FIG. 69.—RIGHT TIBIA AND FIBULA OF HORSE, POSTERIOR VIEW.
]

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.

FIG. 70.—RIGHT TIBIA AND FIBULA OF HORSE, ANTERIOR VIEW.
]

=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.

FIG. 71.—PROXIMAL EXTREMITY OF RIGHT TIBIA OF HORSE, END VIEW.

_l.c.a._, _l.c.p._, Depressions for attachment of anterior and
posterior crucial ligaments; _l_, _m_, _m_, depressions for
attachment of semilunar cartilages.
]

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=.

FIG. 72.—FRONTAL SECTION OF RIGHT TIBIA OF HORSE, ANTERIOR VIEW.
]

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.

FIG. 73.—RIGHT PATELLA OF HORSE, ANTERIOR VIEW.
]

FIG. 74.—RIGHT PATELLA OF HORSE, POSTERIOR VIEW.
]

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

FIG. 75.—RIGHT TARSUS AND UPPER PART OF METATARSUS OF HORSE, INTERNAL
VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 76.—RIGHT TARSUS AND UPPER PART OF METATARSUS OF HORSE, POSTERIOR
(PLANTAR) VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

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.

FIG. 77.—RIGHT TIBIAL TARSAL BONE OF HORSE, ANTERIOR VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 78.—RIGHT TIBIAL TARSAL BONE OF HORSE, EXTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

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.

FIG. 79.—RIGHT FIBULAR TARSAL BONE OF HORSE, ANTERIOR VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 80.—RIGHT FOURTH TARSAL BONE OF HORSE, UPPER SURFACE. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 81.—RIGHT CENTRAL TARSAL BONE OF HORSE, UPPER SURFACE. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 82.—RIGHT FIRST, SECOND, AND THIRD TARSAL BONES OF HORSE, UPPER
SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 83.—PROXIMAL ARTICULAR SURFACES OF LEFT METATARSAL BONES OF
HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

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.

FIG. 84.—RIGHT TARSAL AND METATARSAL BONES OF HORSE. EXTERNAL VIEW.
(After Schmaltz, Atlas d. d. Anat. d. Pferdes.)
]

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.

FIG. 85.—CROSS-SECTIONS OF METACARPAL AND METATARSAL BONES OF HORSE.
(After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

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.

FIG. 86.—DIGITAL BONES OF FORE LIMB OF HORSE, VOLAR ASPECT.
]

FIG. 87.—DIGITAL BONES OF HIND LIMB OF HORSE, VOLAR ASPECT.
]

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.

The =large= or =third metatarsal= bone is about one-sixth longer than the corresponding metacarpal; in an animal of medium size the difference is about two inches. The shaft is more cylindrical, and is almost circular on cross-section, except in its lower part. At the upper part of its external surface there is a groove, which is directed obliquely downward and backward, and is continued by the furrow formed by the apposition of the external metatarsal bone; it indicates the course of the great metatarsal artery. A faint impression in a similar place on the inner side marks the position of the corresponding vein. The nutrient foramen is relatively higher than on the metacarpal bone. The proximal extremity is much wider from before backward than that of the metacarpal bone. Its articular surface is slightly concave, and is marked by a large central non-articular depression, continued outward by a deep notch. The greater part of the surface articulates with the third tarsal, but there is an outer facet for the fourth, and usually a small facet posterointernally for the second tarsal bone. Posteriorly there are two pairs of facets for articulation with the small metatarsal bones. The front is crossed by a rough ridge for insertion, which becomes larger and turns downward on the outer side behind the vascular groove. The distal extremity closely resembles that of the corresponding metacarpal bone.

In some cases the lower part of the shaft is bent backward somewhat.
The articular surface extends a little higher behind than in the case
of the metacarpal bone. The large metatarsal bone is even more
strongly constructed than the metacarpal. The shell of compact
substance is very thick in the middle of the shaft, especially in
front and internally.

The =small metatarsal= bones are a little longer than the corresponding metacarpals. The =external= (fourth) =metatarsal= is relatively massive, especially in its upper part. The head is large and outstanding, and bears two facets above for the fourth tarsal, and two in front and internally for articulation with the large metatarsal; elsewhere it is roughened for attachment. The =internal= (second) =metatarsal= is much more slender than the outer one, especially in its upper part. The head bears two facets above for the first and second tarsals, and sometimes one for the third tarsal.

THE PHALANGES AND SESAMOIDS

The axis of the phalanges of the hind limb is about five degrees less oblique than that of the fore limb, and the chief differences in the form and size of the bones are as follows:

The =first phalanx= is a little shorter, wider above, and narrower below.

The =second phalanx= is narrower and slightly longer.

The =third phalanx= is narrower, the angle of inclination of the wall surface is a little (ca. 5 degrees) greater, the sole surface is more concave, and the wings are less prominent and closer together.

The =proximal sesamoids= are a little smaller, except in thickness. The =third sesamoid= or =navicular bone= is narrower and shorter.

SKELETON OF THE OX

VERTEBRAL COLUMN

The vertebral formula is C_{7}T_{13}L_{6}S_{5}Cy_{18–20}.

The =cervical vertebræ= are much shorter than those of the horse and are smaller in their other dimensions. The =articular processes= are smaller than in the horse, and a plate of bone connects each two of the same side. The transverse processes of the third, fourth, and fifth are double; the upper part projects backward, and is short and stout; the lower part is directed downward and forward, and is longer and more plate-like. The lower part of the sixth transverse process is a large, thick, quadrilateral and almost sagittal plate, directed downward. The seventh transverse process is single, short, and thick, and presents no foramen transversarium; it is in series with the upper part of the preceding processes. The =spinous processes= are well developed, and increase in height from before backward. They are directed upward and forward, with the exception of the last, which is nearly vertical and is about four or five inches (ca. 10 to 12 cm.) in height. The summit of that of the third bone is usually bifid. The =ventral spines= are prominent and thick in their posterior part; they are absent on the last two.

The =atlas= has a large rough =tuberosity= on its dorsal arch. The =ventral arch= is very thick. The =wings= are less curved than in the horse, and the foramen transversarium (posterior foramen) is absent. The cavities for the occipital condyles are divided into upper and lower parts by a non-articular area, and are separated by a narrow interval below. The =posterior articular surfaces= are flattened behind and are continued into the vertebral canal, forming an extensive area for the odontoid process of the axis.

The =axis= is short. The =spine= projects a little in front, and increases in height and thickness behind; its posterior border descends abruptly. The =odontoid process= (dens) is wide, and its upper surface is deeply concave from side to side. The =intervertebral foramen= is circular and not so close to the anterior border of the arch as in the horse. The =posterior notches= are not so deep. The =transverse processes= are stouter, but the =foramen transversarium= is small and sometimes absent.

The =thoracic vertebræ=, thirteen in number, are larger than those of the horse. The =bodies= are longer and are distinctly constricted in the middle. They bear a thin-edged ventral crest. The =arches=—in addition to the usual notches, which are shallow—are perforated in the posterior part by a foramen. The =transverse processes= are thick and strong, and bear rounded mammillary processes (except at the posterior end of the series); the last two, although prominent, do not always articulate with the ribs. The =spinous processes= are long. The first is much higher than in the horse, the next two are usually the most prominent, and behind this there is a very gradual diminution in height. The backward slope, slight at first, increases to the tenth; the last is vertical and lumbar in character. The summit is usually pointed on the first, and the thickening on those further back is less than in the horse. The width diminishes from the fifth to the eleventh usually. Both borders of the spines are in general thin and sharp, but the last three or four sometimes have thick posterior margins.

FIG. 88.—SKELETON OF COW.

_1.H._, Atlas; _7.H._, seventh cervical vertebra; _1.R._, first
thoracic vetrebra; _6.R._, sixth rib; _12.R._, twelfth thoracic
vertebra; _13.R._, last rib; _1.L._, first, _6.L._, last lumbar
vertebra; _K_, sacrum; _1.S._, first coccygeal vertebra; _6.K._,
sixth costal cartilage; _x_, wing of atlas; _1_, scapula; _1′_,
cartilage of scapula; _2_, spine of scapula; _3_, acromion; _4_,
humerus; _4′_, external condyle of humerus; _5_, external tuberosity
of humerus; _6_, deltoid tuberosity; _7_, ulna; _8_, olecranon; _9_,
radius; _10_, carpus; _11_, accessory carpal bone; _12_, metacarpus;
_13_, phalanges; _14_, sternum; _14′_, manubrium; _14″_, xiphoid
cartilage; _15_, ilium; _16_, external angle of ilium; _16′_,
internal angle of ilium; _17_, tuber ischii; _18_, femur; _19_,
trochanter major; _20_, patella; _21_, tibia; _21′_, external
condyle of tibia; _22_, tarsus; _23_, distal end of fibula; _24_,
tuber calcis; _25_, metatarsus; _26_, phalanges. (After
Ellenberger-Baum, Anat. für Künstler.)
]

The =lumbar vertebræ=, six in number, are much longer than in the horse. The =bodies= are much constricted in the middle, expanded at either end, and bear rudimentary ventral crests. The fourth and fifth are usually the longest. The =intervertebral foramina= are often double in the anterior part of the series, and are very large further back. The =articular processes= are large, and their facets are more strongly curved than in the horse. The =transverse processes= all curve forward. They are separated by considerable intervals, and form no articulations with each other or with the sacrum. Their borders are thin and irregular, and often bear projections of variable size and form. The first is the shortest and the length increases to the fifth, the last being considerably shorter. The =spinous processes= are relatively low and wide, the last being the smallest; their summits are moderately thickened.

The =sacrum= is longer than that of the horse. It consists originally of five segments, but fusion is more complete and involves the spinous processes, which are united to form a =median sacral crest=, with a convex thick and rough margin. =A lateral crest= is formed by the fusion of the articular processes. The =pelvic surface= is concave in both directions, and is marked by a central =groove=, which indicates the course of the middle sacral artery. The =inferior sacral foramina= are large. The =wings= are quadrangular, short, compressed from before backward, and high dorso-ventrally. They have an extensive anterior surface, which is concave from side to side, and non-articular. The posterior surface is rough, and bears a triangular area below for articulation with the ilium. The body of the first segment is very wide, and the entrance to the sacral canal correspondingly wide and low. The =anterior articular processes= are large, and are concave and semicylindrical in curvature internally. The =lateral borders= are thin, sharp, and irregular. The =apex= is wider than in the horse, and the posterior end of the crest forms a pointed projection over the opening of the sacral canal.

The =coccygeal vertebræ= are longer and better developed than in the horse. The first five or six have complete arches and spinous processes. The transverse processes are relatively large in the anterior part of the series, in which there are also anterior articular processes (which do not articulate), and a pair of ventral spines which form a hæmal groove for the middle coccygeal artery.

Vertebral curves: The cervical curve is very slight and is concave
dorsally. The thoracic and lumbar regions form a gentle curve, concave
ventrally. The promontory is more pronounced than in the horse,
especially in subjects in which the sacrum is inclined upward behind.
Another promontory occurs at the junction of the sacrum and first
coccygeal vertebræ.

Length: The following table gives the lengths of the regions of a
shorthorn cow of medium size:

Cervical 47 cm.
Thoracic 75 cm.
Lumbar 40 cm.
Sacral 24 cm.
Coccygeal 75 cm.
———————
261 cm.

Variations: Sometimes fourteen thoracic vertebræ and fourteen pairs of
ribs are present; reduction to twelve with the normal number of lumbar
vertebræ is very rare. According to Franck, there are sometimes seven
lumbar vertebræ with the normal number in the thoracic region. The
number of coccygeal vertebræ may vary from sixteen to twenty-one.

THE RIBS

Thirteen pairs of ribs are normally present, of which eight are sternal and five asternal. They are in general longer, wider, flatter, less curved, and less regular in form than in the horse. The eighth, ninth, and tenth are the longest and widest. The width of most of the ribs increases considerably in the middle, and the breadth of the intercostal spaces is correspondingly diminished. The neck is long, and forms (except in the posterior part of the series) a smaller angle with the shaft than in the horse. The articular surface of the tubercle is concave transversely, except on the last two or three, where the facet is small and flat or absent. The lower ends of the second to the tenth or eleventh inclusive form diarthrodial joints with the costal cartilages. The first costal cartilages are very short; they articulate by their internal surfaces with the sternum, but not with each other.

The presence of a fourteenth rib is not very rare. It is usually
floating and may correspond to an additional thoracic vertebra or to
the first lumbar. Reduction of the thirteenth is more common. The
eighth cartilage often does not reach the sternum, but articulates
with the seventh.

The =sternum= consists of seven sternebræ, most of which are developed from two lateral centers. It is wider, flatter, and relatively longer than in the horse, and the ventral crest or “keel” is absent. The manubrium is somewhat wedge-shaped and laterally compressed. Its base forms a diarthrodial joint with the body of the bone, and laterally it bears extensive facets for articulation with the first pair of costal cartilages. The body widens from before backward, but behind the last pair of costal facets it becomes much narrower. The ventral surface is prominent on the second and third segments, concave further back. The lateral borders are notched for the passage of vessels. The cariniform cartilage is absent. The xiphoid cartilage is like that of the horse but is smaller.

THE THORAX

The bony thorax is shorter than in the horse. The inlet is higher. The roof is short, and the floor wider and relatively longer. The transverse diameter is wider in the posterior part. The summits of the spinous processes are almost in a straight line from the second thoracic to the middle of the lumbar region.

BONES OF THE SKULL

BONES OF THE CRANIUM

The =occipital bone= is situated in the lower part only of the posterior surface of the skull, and is separated from the highest part (the frontal eminence) by the parietal and interparietal bones. The supraoccipital, interparietals, and parietals fuse before birth or soon after, and the mass so formed is separated from the lateral parts of the occipital bone by a transverse suture in the skull of the calf. Above this suture is a central tuberosity, to which the ligamentum nuchæ is attached, and the surface on either side is depressed and rough for muscular attachment. Below the suture the bone is much wider than that of the horse. The foramen magnum is wide, so that the condyles are further apart except below. The paramastoid processes are short and wide and are bent inward. Two constant foramina are found in the condyloid fossa; the anterior one is the hypoglossal, the other (sometimes double) conducts a vein from the =condyloid canal=. The latter passes upward from a foramen on the inner side of the condyle and opens into the parieto-temporal canal. The cranial surface of the supraoccipital presents a central depression, and above this is a variable but never very pronounced eminence, which corresponds to the tentorium osseum of the horse. On either side is a groove leading to the parieto-temporal canal. The basilar process is short and wide; its cranial surface is deeply concave, and the internal spheno-occipital crest is prominent. Two large tubercles below mark the junction with the sphenoid. The foramen lacerum is short and very narrow. In the adult animal the bone is excavated to contain an air-cavity which is regarded as a part of the frontal sinus.

The =sphenoid bone= is short. The cranial surface of the body presents a deep sella turcica, in front of which it rises abruptly. The high anterior part bears a central ridge (Rostrum sphenoidale), which joins the crista galli of the ethmoid. Two foramina occur on either side. Of these, the large anterior one is equivalent to the foramen rotundum, lacerum orbitale, and patheticum of the horse. The posterior one is the =foramen ovale=, which transmits the inferior maxillary nerve. The orbital wing is thick and is overlapped by the frontal in such manner as to appear externally to divide into two branches; the anterior part joins the ethmoid close to the sphenopalatine foramen, and contains a small sinus which communicates with an ethmoidal meatus. The temporal wing is small, but forms a prominent thick pterygoid crest. The pterygoid process is wide, and the pterygoid or alar foramen is absent. The sphenoidal sinus is absent in the calf and small in the adult; it communicates by one or two small openings with an ethmoidal meatus, and so with the nasal cavity.

FIG. 89.—SKULL AND ATLAS OF OX, LATERAL VIEW.

_9_, Zygomatic arch; _11_, coronoid process; _12_, supraorbital
process; _13″_, paramastoid or styloid process; _13‴_, occipital
condyle; _14_, parietal bone; _15_, frontal bone; _16_, squamous
temporal bone; _17_, external auditory meatus; _18_, temporal
condyle; _19_, orbital surface of lacrimal bone; _20_, malar bone;
_21_, facial surface of lacrimal bone; _22_, nasal bone; _23_, nasal
process of premaxilla; _24′_, incisor teeth; _26_, maxilla; _27_,
facial tuberosity; infraorbital foramen in front of _27_; _28′_,
molar part of ramus of mandible; _30_, broad vertical part of ramus;
_30′_, angle of jaw; _31_, condyle of mandible; _32_, atlas; _x_,
wing of atlas. (After Ellenberger-Baum, Anat. für Künstler.)
]

The =ethmoid bone= has an extensive perpendicular plate. The lateral mass consists of five endoturbinals and eighteen ectoturbinals (Paulli). The largest ethmoturbinal is so extensive as to be termed a third or middle turbinal bone; it projects forward between the upper and lower turbinals. The lamina papyracea appears to a small extent externally in the pterygo-palatine fossa, forming part of the upper margin of the sphenopalatine foramen.

The =interparietals= are primitively paired, but unite before birth. As already mentioned, fusion occurs before or shortly after birth with the parietals and supraoccipital. No distinct tentorium osseum is present.

FIG. 90.—SKULL OF OX, BASAL VIEW, WITHOUT MANDIBLE.