Chapter VI: Introduction (4)

FIG. 38.—CROSS-SECTION OF NASAL REGION OF SKULL OF HORSE; THE SECTION
IS CUT ABOUT HALFWAY BETWEEN THE ORBIT AND THE ANTERIOR END OF THE
FACIAL CREST, AND PASSES BETWEEN THE FIFTH AND SIXTH CHEEK TEETH.

_a_, Superior, _b_, inferior turbinal bone; _c_, _d_, cavities of _a_
and _b_; _e_, common meatus; _f_, superior, _g_, middle, _h_,
inferior meatus; _i_, placed over ridge in maxillary sinus; _k_,
communication between outer and inner (turbinal) part of maxillary
sinus; _l_, naso-maxillary opening; _m_, naso-lacrimal canal; _n_,
infraorbital canal. (After Ellenberger, in Leisering’s Atlas.)
]

The posterior extremity or fundus is separated from the cranial cavity by the cribriform plate of the ethmoid, and is largely occupied by the lateral masses of that bone.

THE PARANASAL SINUSES

Connected directly or indirectly with the nasal cavity, of which they are diverticula, are four pairs of air-sinuses (Sinus paranasales), viz., maxillary, frontal, sphenopalatine, and ethmoidal.

The =maxillary sinus= (Sinus maxillaris) is the largest. Its external wall is formed by the maxilla, the lacrimal, and the malar. It is bounded internally by the maxilla, the inferior turbinal, and the lateral mass of the ethmoid bone. It extends backward to a transverse plane in front of the root of the supraorbital process, and its anterior limit is indicated approximately by a line drawn from the anterior end of the facial crest to the infraorbital foramen. Its upper boundary corresponds to a line drawn backward from the supraorbital foramen parallel to the facial crest. The floor is formed by the alveolar part of the maxilla; it is very irregular and is crossed by bony plates running in various directions. The last three cheek teeth project up into the cavity to an extent which varies with age; they are covered by a thin plate of bone. The cavity is divided into anterior and posterior parts by an oblique =septum=. The outer margin of the septum is commonly about one and a half to two inches (ca. 3.5 to 5 cm.) from the anterior end of the facial crest; from here it is directed inward, backward, and upward. The upper part of the septum (formed by the posterior end of the inferior turbinal bone) is very delicate and usually cribriform.

The position of the septum is quite variable. It is not rare to find
it further forward, and in some cases it is further back than is
stated above. In the recent state, _i. e._, when covered by the mucous
membrane on both surfaces, it is nearly always complete, but in very
exceptional cases there is an opening of variable size in the upper
part.

The =anterior compartment=, often called the =inferior maxillary sinus=, is partially divided by the infraorbital canal into an external maxillary part and an internal smaller turbinal part. The latter communicates with the middle meatus by a very narrow slit situated at its highest part. The =posterior compartment=, often called the =superior maxillary sinus=, is also crossed by the infraorbital canal, internal to which it opens freely into the sphenopalatine sinus. It communicates dorsally with the frontal sinus through the large oval =fronto-maxillary opening=, situated at the level of the osseous lacrimal canal and the corresponding part of the inner wall of the orbit; the orifice is commonly about one and a half to two inches (ca. 4 to 5 cm.) long and an inch or more (2 to 3 cm.) wide. Just in front of this, and covered by a thin plate, is the narrow =naso-maxillary fissure= (Aditus naso-maxillaris), by which the sinus opens into the posterior part of the middle meatus.

The foregoing statements refer to the arrangement in the average adult
animal. In the foal the cavity (with the exception of its turbinal
part) is largely occupied by the developing teeth. In horses five to
six years of age the maxillary part of the sinus is still filled up to
a large degree by the embedded parts of the teeth. As the teeth are
extruded to compensate the wear, more and more of the cavity becomes
free, until in old age only the short roots project up in the floor,
covered by a layer of bone. Other facts in this connection will be
given in the description of the teeth. In exceptional cases the
posterior part of the inferior turbinal is smaller than usual and
leaves a considerable interval, through which the maxillary sinus
communicates with the nasal cavity.

FIG. 39.—SKULL OF HORSE, LATERAL VIEW WITHOUT MANDIBLE. THE SINUSES
ARE OPENED UP.

_a_, Posterior part, _b_ and _c_, anterior part of frontal sinus; _d_,
roof of superior meatus; _e_, lateral mass of ethmoid bone; _f_,
_f′_, naso-lacrimal duct, exposed in its posterior part; _g_, _h_,
posterior and anterior compartments of maxillary sinus (also
designated as superior and inferior maxillary sinuses); _i_, septum
between _g_ and _h_; _k_, lower limit of upper thin and partly
membranous portion of septum; _l_, infraorbital canal; _m_, turbinal
part of maxillary sinus; _n_, bullous prominence of inferior
turbinal; _o_, orbit; _p_, infraorbital foramen; _q_, continuation
of infraorbital canal to premaxilla; _r_, limit of maxillary sinus.
(After Ellenberger, in Leisering’s Atlas.)
]

The =frontal sinus= (Sinus concho-frontalis) consists of frontal and turbinal parts. The frontal part is bounded chiefly by the two plates of the frontal bone, but its floor is formed in part by the lateral mass of the ethmoid. It extends forward to a plane through the anterior margins of the orbits, backward to one through the temporal condyles, and outward into the root of the supraorbital process. It is separated from the sinus of the opposite side by a complete =septum=. It is partially subdivided by a number of bony plates. The turbinal part is situated in the posterior part of the superior turbinal bone, roofed in by the nasal and lacrimal bones. It extends forward to a transverse plane about half-way between the anterior margin of the orbit and the end of the facial crest. Behind it is in free communication with the frontal part over the lateral mass of the ethmoid. It is separated from the nasal cavity by the thin turbinal plate. The frontal and maxillary sinuses communicate through the large opening described above.

The =sphenopalatine sinus= (Sinus sphenopalatinus) consists of two parts which communicate under the lateral mass of the ethmoid. The sphenoidal (posterior) part is excavated in the body of the presphenoid. The palatine (anterior) part is between the two plates of the perpendicular part of the palate bone, under the lateral mass of the ethmoid; it communicates freely with the maxillary sinus. The septum between the right and left sinuses is not usually median in the sphenoidal part.

In about one-third of the cases (according to Paulli) the sphenoidal
and palatine parts are separated by a transverse septum, and the
sphenoidal part then communicates only with the lower ethmoidal
meatuses.

The term =ethmoidal sinus= is often applied to the cavity of the largest ethmoturbinal. It communicates with the maxillary sinus.

FIG. 40.—SKULL OF HORSE, DORSAL VIEW, WITH SINUSES EXPOSED BY REMOVAL
OF THE OUTER PLATE OF BONE.

_1_, Frontal bone; _2_, nasal bone; _3_, lacrimal bone; _4_, maxilla;
_a_, posterior part of frontal sinus; _a′_, middle part of frontal
sinus; _b_, anterior (turbinal) part of frontal sinus; _c_, lateral
mass of ethmoid bone; _d_, roof of superior meatus; _e_,
fronto-maxillary opening; _f_, naso-maxillary opening below plate
which forms the anterior margin of _e_; _g_, _h_, posterior and
anterior compartments of maxillary sinus—often called the superior
and inferior maxillary sinuses; _i_, septum between _g_ and _h_;
_k_, orbit; _l_, point at which superior turbinal bone may be
perforated to obtain drainage into nasal cavity. (After Ellenberger,
in Leisering’s Atlas.)
]

THE BONES OF THE THORACIC LIMB THE SCAPULA

The =scapula= is a flat bone, situated on the anterior part of the lateral wall of the thorax, and extending obliquely from the vertebral end of the seventh or eighth rib to the sternal end of the first rib. It is curved slightly and slopes outward in adaptation to the form of the thoracic wall. It is triangular in outline, and has two surfaces, three borders, and three angles.

The =external surface= or =dorsum= (Facies lateralis s. dorsalis) is divided into two fossæ by the =spine= (Spina scapulæ), which extends from the vertebral border to the neck of the bone, where it gradually subsides. The free edge of the spine is thick, rough, and in great part subcutaneous. A little above its middle is a variable prominence, the =tubercle= of the spine (Tuber spinæ), to which the trapezius muscle is attached. The =supraspinous fossa= (Fossa supraspinata) is situated in front of the spine, and the =infraspinous fossa= (Fossa infraspinata) behind it. The former is much the smaller of the two; it is smooth and is occupied by the supraspinatus muscle. The infraspinous fossa lodges the infraspinatus muscle; it is wide and smooth in its upper part, narrower below, where it is marked by several rough lines for the attachment of the teres minor muscle; near the neck is the nutrient foramen, and a little lower is a vascular groove.

FIG. 41.—LEFT SCAPULA OF HORSE, EXTERNAL VIEW. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]

The =costal surface= or =venter= (Facies costalis) is hollowed in its length by the =subscapular fossa= (Fossa subscapularis); this occupies nearly the whole of the lower part of the surface, but is pointed above and separates two rough triangular areas (Facies serrata), to which the serratus magnus is attached. In the lower third there is a vascular furrow with several branches.

The =anterior= or =cervical border= (Margo cranialis) is convex and rough above, concave and smooth below.

The =posterior= or =dorsal border= (Margo caudalis) is slightly concave. It is thick and rough in its upper third, thin in its middle, and thickens again below.

The =superior= or =vertebral border= (Margo dorsalis s. basis) carries the =scapular cartilage= (Cartilago scapulæ). In the young subject the edge of the bone is thick, and is pitted by impressions into which the cartilage fits. The cartilage is the unossified part of the fœtal scapula. Its lower edge fits the depressions and elevations of the bone. It thins out toward the free edge, which is convex and lies alongside of the upper parts of the vertebral spines. In front it continues the line of the scapula, but behind it forms a rounded projection. The lower part of the cartilage undergoes more or less ossification, so that the vertebral border of the bone in old subjects is thin, irregular, and porous.

FIG. 42.—LEFT SCAPULA OF HORSE, COSTAL SURFACE.—(After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]

The =anterior= or =cervical angle= (Angulus cranialis) is at the junction of the anterior and vertebral borders and lies opposite to the second thoracic spine. It is relatively thin and is about a right angle.

The =posterior= or =dorsal angle= (Angulus caudalis) is thick and rough; its position can be determined readily in the living animal.

The =inferior= or =articular angle= (Angulus glenoidalis) is joined to the body of the bone by the =neck= of the scapula (Collum scapulæ). It is enlarged, especially in the sagittal direction. It bears the =glenoid cavity= (Cavitas glenoidalis) for articulation with the head of the humerus. The cavity is oval in outline, and its margin is cut into in front by the =glenoid notch= (Incisura glenoidalis), and is rounded off externally; just above its postero-external part is a tubercle to which a tendon of the teres minor is attached. The =bicipital tuberosity= or =tuber scapulæ= is the large rough prominence in front, to which the tendon of origin of the biceps brachii is attached; projecting from its inner side is the small =coracoid process= (Processus coracoideus), from which the coraco-brachialis muscle arises.

=Development.=—The scapula has four centers of ossification, viz., one each for the body of the bone, the bicipital and coracoid processes, the anterior part of the glenoid cavity, and the tuber spinæ. The last ossifies after birth and fuses with the spine about the third year. The bicipital tuberosity and coracoid fuse with the body of the bone about the end of the first year.

In old subjects the spongy substance disappears at the middle part of
the fossæ, so that the bone consists here of a thin layer of compact
substance. Considerable ossification of the cartilage is usual, the
borders become much rougher, the muscular lines are more pronounced,
and a medullary cavity may appear in the neck. Much variation occurs
in dimensions and slope. The average ratio between the length and
breadth (scapular index) is about 1 ∶ 0.5, but in many cases the base
is relatively wider. The inclination on a horizontal plane varies from
50 to 65 degrees. Exceptionally the coracoid process reaches a length
of an inch or more (2½ to 3 cm.), and the chief nutrient foramen may
be on the posterior border or in the subscapular fossa.

FIG. 43.—DISTAL END OF LEFT SCAPULA OF HORSE, END VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

THE HUMERUS

The =humerus= is a long bone which extends from the shoulder above, where it articulates with the scapula, to the elbow below and behind, where it articulates with the radius and ulna. It is directed obliquely downward and backward, forming an angle of about 55 degrees with a horizontal plane. It may be divided into a shaft and two extremities.

FIG. 44.—PROXIMAL END OF LEFT HUMERUS OF HORSE, END VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The =shaft= or =body= (Corpus humeri) is irregularly cylindrical and has a twisted appearance. It may be regarded as having four surfaces. The =external surface= is smooth and is spirally curved, forming the =musculo-spiral groove= (Sulcus musculi brachialis), which contains the brachialis muscle; the groove is continuous with the posterior surface above and winds around toward the front below. The =internal surface= is nearly straight in its length, rounded from side to side, and blends with the anterior and posterior surfaces. Just above its middle is the =internal= or =teres tubercle= (Tuberositas teres), to which the tendon of the latissimus dorsi and teres major muscles is attached. The =nutrient foramen= is in the lower third of this surface. The =anterior surface= is triangular, wide and smooth above, narrow and roughened below. It is separated from the external surface by a distinct border, which bears on its upper part the =deltoid tuberosity= (Tuberositas deltoidea). From the latter a rough line curves upward and backward to the outer surface of the neck, and gives origin to the external head of the triceps muscle. Below the tuberosity the border inclines forward, becomes less salient, and ends at the coronoid fossa. The =posterior surface= is rounded from side to side and smooth.

FIG. 45.—LEFT HUMERUS OF HORSE, INTERNAL SURFACE. (After Schmaltz,
Atlas d. Anat. d. Pferdes.)
]

FIG. 46.—LEFT HUMERUS OF HORSE, EXTERNAL SURFACE. (After Schmaltz,
Atlas d. Anat. d. Pferdes.)
]

The =proximal extremity= consists of the head, neck, two tuberosities, and the bicipital groove. The =head= (Caput humeri) presents an almost circular convex articular surface, which is about twice as extensive as the glenoid cavity of the scapula, with which it articulates. In front of the head is a fossa, in which are several foramina. The neck (Collum humeri) is well defined behind, but is practically absent elsewhere. The =external tuberosity= (Tuberculum majus) is placed antero-externally, and consists of two parts; the anterior part forms the outer boundary of the bicipital groove and gives attachment to the external branch of the supraspinatus muscle; the posterior part gives attachment to the short insertion of the infraspinatus, while its outer surface is coated with cartilage, over which the chief tendon of the same muscle passes to be inserted into a triangular facet on the outer aspect of the anterior part. The =internal tuberosity= (Tuberculum minus) is less salient, and consists of anterior and posterior parts; the anterior part forms the inner boundary of the bicipital groove, and furnishes insertion to the inner branch of the supraspinatus above, and the posterior deep pectoral muscle below; the posterior part gives attachment to the subscapularis muscle. The =bicipital= or =intertubercular groove= (Sulcus intertubercularis) is situated in front; it is bounded by the anterior parts of the tuberosities, and is subdivided by an intermediate ridge. The groove is covered in the fresh state by cartilage, and lodges the tendon of origin of the biceps brachii muscle. Just below the intermediate ridge is a small fossa in which several foramina open.

FIG. 47.—LEFT HUMERUS OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]

The =distal extremity= has an oblique surface for articulation with the radius and ulna, which consists of two condyles of very unequal size, separated by a ridge. The =internal condyle= (Condylus medialis) is much the larger, and is crossed by a sagittal groove, on the anterior part of which there is usually a synovial fossa. Posteriorly the groove extends upward considerably above the rest of the articular surface and reaches the olecranon fossa, and this part articulates with the semilunar notch of the ulna. The =external condyle= (Condylus lateralis) is much smaller and is placed somewhat lower and further back, giving the extremity an oblique appearance; it is marked by a wide shallow groove. The =coronoid fossa= (Fossa coronoidea) is situated in front above the groove on the internal condyle; it furnishes origin to part of the extensor carpi, and external to it is a rough depression from which the anterior or common extensor of the digit arises. Behind and above the condyles are two thick ridges, the epicondyles. The =internal= or =flexor epicondyle= (Epicondylus medialis s. flexorius) is the more salient; it furnishes origin to flexor muscles of the carpus and digit, and presents internally a tubercle for the attachment of the internal lateral ligament of the elbow joint. The =external= or =extensor epicondyle= (Epicondylus lateralis s. extensorius) bears externally the =external supracondyloid crest= (Crista condyloidea lateralis), which forms here the outer boundary of the musculo-spiral groove, and gives origin to the extensor carpi. Below this is a rough excavation in which the external lateral ligament is attached. The lower border of the epicondyle gives attachment to the flexor carpi externus. Between the epicondyles is the deep =olecranon fossa= (Fossa olecrani).

=Development.=—The humerus ossifies from six centers, viz., three primary centers for the shaft and epiphyses, and three secondary centers for the external tuberosity, the deltoid tuberosity, and the internal condyle. The proximal end fuses with the shaft at about three and one-half years, the distal at about one and a half years of age.

THE RADIUS

The =radius= is much the larger of the two bones of the forearm in the horse. It extends in a vertical direction from the elbow, where it articulates with the humerus, to the carpus below. It is gently curved, the convexity being anterior. It consists of a shaft and two extremities.

The =shaft= (Corpus radii) is curved in its length, somewhat flattened from before backward, and expanded at its ends. It presents for description two surfaces and two borders. The =anterior surface= (Facies dorsalis) is smooth, slightly convex in its length, and rounded from side to side. The =posterior surface= (Facies volaris) is correspondingly concave in its length and is flattened in the transverse direction. At its upper part there is a smooth shallow groove, which concurs with the ulna in the formation of the =interosseous space= of the forearm; the nutrient foramen is in the lower part of this groove. Below this there is in the young subject a narrow, rough, triangular area to which the ulna is attached by an interosseous ligament; in the adult the two bones are fused here. A variable rough elevation below the middle of the surface and close to the internal border gives attachment to the superior check ligament. The =internal border= (Margo medialis) is slightly concave in its length and is largely subcutaneous; at its proximal end there is a smooth area on which the tendon of insertion of the brachialis muscle lies, and a small rough area just below gives attachment to that muscle and the long internal lateral ligament of the elbow joint. The =external border= (Margo lateralis) is more strongly curved, but presents no special features.

The =proximal extremity= or =head= (Capitulum radii) is flattened from before backward and wide transversely. It presents an articular surface (Fovea capituli) which corresponds to that on the distal end of the humerus; it is crossed by a central sagittal ridge, which has a synovial fossa on its posterior part, and ends in front at a prominent lip, the =coronoid process= (Processus coronoideus). Just below the posterior border there are two concave facets for articulation with the ulna, and between these and the interosseous space is a quadrilateral rough area at which the two bones are united by an interosseous ligament. At the inner side of the anterior surface is the =bicipital tuberosity= (Tuberositas radii), into which the biceps tendon is inserted. The =internal tuberosity= is continuous with the preceding eminence, and furnishes attachment to the short part of the internal lateral ligament. The =external tuberosity= is more salient; it gives attachment to the external lateral ligament and to the anterior and lateral extensor muscles of the digit.

FIG. 48.—LEFT RADIUS AND ULNA OF HORSE, EXTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 49.—LEFT RADIUS AND ULNA OF HORSE, POSTERIOR VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The =distal extremity= is also compressed from before backward. It presents the =carpal articular surface= (Facies articularis carpea) which consists of three parts. The inner facet is the largest, is quadrilateral, concavo-convex from before backward, and articulates with the radial carpal bone (or scaphoid); the middle one is somewhat similar in form but smaller, and articulates with the intermediate carpal bone (or semilunar); the outer facet is smaller, is convex, and articulates below with the ulnar carpal (or cuneiform) and behind with the accessory carpal (or pisiform). The anterior surface presents three =grooves=, separated by ridges. The middle one is vertical and gives passage to the tendon of the extensor carpi radialis; the outer one is similar and contains the tendon of the anterior extensor of the digit; the inner one is small and oblique and lodges the tendon of the extensor carpi obliquus. The posterior aspect is crossed by a rough ridge, below which are three depressions. On either side is a tuberosity (Tuberculum ligamenti) to which the lateral ligament is attached. The outer one is marked by a small vertical groove for the passage of the lateral extensor tendon.

=Development.=—The radius ossifies from four centers, viz., one each for the shaft, the two extremities, and the outer part of the distal end; the last is morphologically the distal end of the ulna which has fused with the radius, and the line of fusion is often indicated by a distinct groove on the carpal articular surface. The proximal extremity unites with the shaft at about one and a half years, the distal end at about three and a half years.

FIG. 50.—UPPER HALF OF RADIUS AND ULNA OF HORSE, INTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

THE ULNA

The =ulna= of the horse is a reduced long bone situated behind the radius, with which it is partially fused in the adult.

The =shaft= (Corpus ulnæ) is three-sided and tapers to a point below. The =anterior surface= (Facies dorsalis) is applied to the posterior surface of the radius, and below the interosseous space the two bones are fused in the adult. The surface which enters into the formation of the space is smooth and usually presents a small nutrient foramen, directed upward. Above the space it is rough and is attached to the radius by an interosseous ligament which is usually permanent. The =internal surface= (Facies medialis) is smooth and slightly concave. The =external surface= (Facies lateralis) is flattened. The =internal= and =external borders= are thin and sharp, except at the interosseous space. The =posterior border= is slightly concave in its length and is rounded. The lower end is pointed and is usually a little below the middle of the radius. It is commonly continued by a fibrous cord to the distal external tuberosity of the radius, but this band may be replaced in part or entirely by bone.

The =proximal extremity= is the major part of the bone. It projects upward and somewhat backward behind the lower end of the humerus, and forms a lever arm for the extensor muscles of the elbow. The =internal surface= is concave and smooth. The =external surface= is convex and is roughened above. The =anterior border= bears on its middle a pointed projection, the =processus anconæus= or “beak,” which overhangs the =semilunar notch= or =sigmoid cavity= (Incisura semilunaris). The latter is triangular in outline, concave from above downward, and articulates with the humerus; in the middle of its lower part is an extensive synovial fossa. Just below the notch are two convex =facets= which articulate with those on the posterior aspect of the proximal end of the radius. The =posterior border= is nearly straight, and is thick and rounded. The free end or summit is a rough tuberosity, the =olecranon=, which gives attachment to the triceps brachii and other muscles.

The primitive =distal extremity= has, as previously stated, fused with the radius.

=Development.=—The ulna ossifies from three centers, of which one is for the main part of the bone, one for the olecranon, and one for the distal end. The cartilaginous embryonic ulna extends the entire length of the forearm. The lower part of the shaft is usually reduced to a small fibrous band or may disappear entirely; in some cases a variable remnant of it ossifies. The distal extremity fuses early with the radius. The olecranon unites with the rest of the bone at three to three and a half years. A medullary canal appears to occur constantly in the adult—contrary to the statements of some authors.

THE CARPUS

The =carpus= of the horse consists of seven or eight bones (Ossa carpi) arranged in two rows, proximal or antibrachial, and distal or metacarpal. The (abbreviated) names and relative positions of the bones of the left carpus as seen from in front are indicated below.

_Proximal Row:_
Radial Intermediate Ulnar Accessory

_Distal Row:_
First Second Third Fourth

THE RADIAL CARPAL BONE

The =radial carpal= bone (Os carpi radiale, scaphoid) is the largest bone of the upper row; it is somewhat compressed laterally, and is clearly six-sided. The =superior= or =proximal surface= is convex in front, concave behind, and articulates with the inner facet on the distal end of the radius. The =inferior= or =distal surface= is also convex in front and concave behind; it articulates with the second and third carpal bones. The =external surface= bears upper and lower facets on its anterior part for articulation with the intermediate; between and behind these it is excavated and rough. The =anterior= or =dorsal surface= is rough and slightly convex. The =internal surface= and the =posterior= or =volar surface= are rough and tuberculate.

FIG. 51.—SAGITTAL SECTION OF UPPER PART OF RADIUS AND ULNA OF HORSE.
_Cm_, Medullary cavity of ulna.
]

THE INTERMEDIATE CARPAL BONE

The =intermediate carpal= bone (Os carpi intermedium, semilunar, lunar) is somewhat wedge-shaped, wider in front than behind. The =superior= or =proximal surface= is saddle-shaped, and articulates with the middle facet on the distal end of the radius. The =inferior= or =distal surface= is smaller, convex in front, concave behind, and articulates with the third and fourth carpal bones. The =internal surface= has upper and lower facets for articulation with the radial carpal, and between these it is excavated and rough. The =external surface= is similar to the preceding and articulates with the ulnar carpal. The =anterior= or =dorsal surface= is rough and slightly convex. The =posterior= or =volar surface= bears a tuberosity on its lower part.

THE ULNAR CARPAL BONE

The =ulnar carpal= bone (Os carpi ulnare, cuneiform) is the smallest and most irregular bone of the upper row. The =superior= or =proximal surface= is concave, and fits the lower part of the outer facet on the distal end of the radius. The =inferior= or =distal surface= is oblique and undulating for articulation with the fourth carpal bone. The =internal surface= has upper and lower facets for articulation with the intermediate. The =anterior= or =dorsal= and =external surfaces= are continuous, convex, and rough. The =posterior= or =volar surface= is oblique, and bears a concave facet for articulation with the accessory carpal bone; below this is a tubercle.

FIG. 52.—LEFT CARPAL BONES OF HORSE, WITH DISTAL END OF RADIUS AND
PROXIMAL END OF METACARPUS; INTERNAL VIEW.
]

FIG. 53.—LEFT CARPAL BONES OF HORSE, WITH ADJACENT ENDS OF RADIUS AND
METACARPUS; EXTERNAL VIEW.

_Ca_, Accessory carpal bone; _Cr_, radial carpal; _Ci_, intermediate
carpal; _Cu_, ulnar carpal; _C1–4_, first to fourth carpals; _Mc.
II_, _III_, _IV_, metacarpal bones; _1_, groove for tendon of
extensor carpi obliquus; _2_, groove for lateral extensor tendon;
_3_, groove for tendon of flexor carpi externus; _4_, metacarpal
tuberosity. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

THE ACCESSORY CARPAL BONE

The =accessory carpal= bone (Os carpi accessorium, pisiform) is situated behind the ulnar carpal bone and the outer part of the distal end of the ulna. It is discoid and presents for description two surfaces and a circumference. The =internal surface= is concave and forms the outer wall of the carpal groove. The =external surface= is convex and rough; a smooth groove for the outer tendon of the flexor carpi externus crosses its anterior part obliquely downward and slightly forward. The =anterior border= bears two facets; the upper one is concave and articulates with the back of the outer facet on the distal end of the radius; the lower one is convex and articulates with the ulnar carpal bone. The remainder of the circumference is rounded and rough.

The accessory does not directly bear weight, and may be regarded as a
sesamoid bone interposed in the course of the tendons of the middle
and external flexors of the carpus, which it enables to act at a
mechanical advantage. The posterior border furnishes attachment to the
transverse carpal ligament, which completes the carpal canal for the
flexors of the digit.

THE FIRST CARPAL BONE

The =first carpal= bone (Os carpale primum, trapezium) is a small inconstant bone, commonly about the size and shape of a pea, which is situated in the lower part of the internal lateral ligament behind the second carpal bone.

This bone appears to be absent on both sides in about half of the
cases; in a good many subjects it is present on one side only. In size
it varies from a minute nodule to a discoid mass 10 to 12 mm. in
length. In exceptional cases it articulates with both the second
carpal and the second (inner) metacarpal bone, in other cases with the
former only, but in the majority of specimens no articular facet is
present.

THE SECOND CARPAL BONE

The =second carpal= bone (Os carpale secundum, trapezoid) is the smallest constant bone of the lower row, and is irregularly hemispherical in shape. The =superior= or =proximal surface= is a convex facet which is continued upon the =posterior= or =volar surface= and articulates with the posterior part of the radial carpal. The =external surface= faces obliquely outward and forward, and bears three facets for articulation with the third carpal bone. The =anterior= or =dorsal= and the =internal surface= bear a tuberosity to which the lateral ligament is attached. The =inferior= or =distal surface= is articular and consists of a large flattened facet for the inner (second) metacarpal bone, and a small one for the large (third) metacarpal bone. Some specimens have a small facet on the lower part of the posterior surface which articulates with the first carpal bone.

FIG. 54.—CARPAL BONES OF HORSE, WITH ADJACENT ENDS OF RADIUS AND
METACARPUS; ANTERIOR VIEW. THE ACCESSORY AND FIRST CARPAL BONES ARE
NOT SHOWN.
]

FIG. 55.—CARPAL ARTICULAR SURFACE OF RADIUS AND PROXIMAL ARTICULAR
SURFACES OF CARPAL AND METACARPAL BONES, LEFT SIDE. THE ACCESSORY
AND FIRST CARPAL BONES ARE NOT SHOWN.

_Cr_, Radial carpal; _Ci_, intermediate carpal; _Cu_, ulnar carpal;
_C2_, _C3_, _C4_, second, third, and fourth carpals; _Mc.II_, second
or inner small metacarpal bone; _Mc.III_, third or large metacarpal
bone; _Mc.IV_, fourth or outer small metacarpal bone; _1_, _2_,
grooves for tendons of anterior extensor and extensor carpi
radialis; _3_, metacarpal tuberosity. Arrows indicate relations of
facets. Short arrow points to facet on ulnar carpal for articulation
with accessory carpal. (After Schmaltz, Atlas. d. Anat. d. Pferdes.)
]

THE THIRD CARPAL BONE

The =third carpal= bone (Os carpale tertium, os magnum) is much the largest bone of the lower row, forming more than two-thirds of the width of the latter. It is flattened from above downward, and is twice as wide in front as behind. The =superior= or =proximal surface= consists of two facets separated by an antero-posterior ridge; the inner facet is concave and articulates with the radial carpal; the outer facet—for the intermediate carpal—is concave in front and convex behind, where it encroaches on the posterior surface. The =inferior= or =distal surface= is slightly undulating, and articulates almost entirely with the large (third) metacarpal bone, but it usually bears a small oblique facet at its inner side for the inner (second) metacarpal, and there is commonly a non-articular depression externally. The _internal surface_ faces backward and inward, and bears three facets for articulation with the second carpal, between which it is excavated and rough. The =external surface= has two facets for articulation with the fourth carpal, and is depressed and rough in its middle. The =anterior= or =dorsal surface= is convex and is crossed by a rough transverse ridge. The =posterior= or =volar surface= is relatively small, and is rounded; its upper part is encroached upon by the superior articular surface, below which it is rough.

THE FOURTH CARPAL BONE

The =fourth carpal= bone (Os carpale quartum, unciform) is somewhat wedge-shaped, and is readily distinguished from the second by its greater size and its posterior tubercle. The =superior= or =proximal surface= articulates with the intermediate and ulnar; it is convex and curves outward, backward, and downward, encroaching on the external and posterior surfaces. The =inferior= or =distal surface= bears two inner facets for the large (third) metacarpal and an outer one for the external (fourth) metacarpal bone. The =internal surface= has two or three facets for articulation with the third carpal, between which it is excavated and rough. The =anterior= or =dorsal surface= is convex and rough. The =external surface= is small, being encroached upon by the superior articular surface. The =posterior= or =volar surface= bears a tubercle on its lower part.[21]

THE CARPUS AS A WHOLE

The bones of the carpus, exclusive of the accessory, form an irregular quadrangular mass, the width of which is about twice the height or the antero-posterior diameter. The =anterior= or =dorsal surface= is convex from side to side, depressed along the line of junction of the two rows, and prominent below. The =posterior= or =volar surface= is in general slightly convex, but very irregular. It forms with the accessory the =carpal groove= (Sulcus carpi), which in the recent state is rendered smooth by the posterior ligament; it is converted into the =carpal canal= (Canalis carpi) for the flexor tendons by the transverse carpal ligament, which stretches across from the accessory bone to the inner side. The =proximal surface= is widest internally and is elevated in front, concave behind; it is entirely articular and adapted to the carpal articular surface of the radius. The =distal surface= is also articular and is irregularly faceted in adaptation to the surfaces of the metacarpal bones; each of the lower bones usually articulates with two metacarpal bones, but sometimes the third does not bear on the inner metacarpal bone. The =lateral surfaces= are both irregular and rough, the internal one being the wider. With the exception of the accessory, ulnar, and second, each bone articulates with two bones of the other row.

=Development.=—Each ossifies from a single center.

THE METACARPUS

Three =metacarpal bones= (Ossa metacarpalia) are present in the horse. Of these, only one, the third or large metacarpal bone, is fully developed and carries a digit; the other two, the second and fourth, are much reduced, and are commonly called the internal and external small metacarpal or “splint” bones.

FIG. 56.—DISTAL ROW OF CARPAL BONES, METACARPUS, FIRST AND SECOND
PHALANGES, AND PROXIMAL SESAMOID BONES OF HORSE; POSTERIOR VIEW,
LEFT SIDE.

The fourth (external) metacarpal bone is wrongly numbered as I. (After
Schmaltz, Atlas. d. Anat. d. Pferdes.)
]

FIG. 57.—LEFT METACARPAL AND DIGITAL BONES OF HORSE, INTERNAL VIEW.
(After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

THE LARGE METACARPAL BONE

This (Os metacarpale tertium) is a very strong long bone, placed vertically between the carpus above and the first phalanx below. It consists of a shaft and two extremities.

The =shaft= (Corpus) is semicylindrical, and presents two surfaces and two borders. The =anterior= or =dorsal surface= is smooth, convex from side to side, and nearly straight in its length. The =posterior= or =volar surface= is somewhat convex from side to side and, with the small bones, forms a wide groove which lodges the suspensory ligament. On either side of its upper two-thirds it is roughened for the attachment of the small metacarpal bones. The =nutrient foramen= occurs at the junction of the upper and middle thirds. The lower third is wider and flattened. The borders are rounded.

The =proximal extremity= (Basis) bears an undulating articular surface adapted to the lower row of carpal bones. The greater part supports the third carpal bone; the oblique outer part, separated from the preceding by a ridge, articulates with the fourth, and a small facet for the second is usually found at the postero-internal angle. On either side is a notch separating two small facets which articulate with the proximal ends of the small metacarpal bones. Toward the inner side of the anterior surface is the =metacarpal tuberosity=, into which the extensor carpi radialis is inserted. The posterior surface is roughened for the attachment of the suspensory ligament.

The =distal extremity= (Trochlea s. Capitulum) presents an articular surface for the first phalanx and the proximal sesamoid bones, which is composed of two condyles, separated by a sagittal ridge; the inner condyle is slightly the larger. On either side is a small fossa, surmounted by a tubercle, for the attachment of the lateral ligaments of the fetlock joint.

The large metacarpal is one of the strongest bones in the skeleton.
The compact substance is specially thick in front and internally. The
medullary canal extends further toward the ends than in most of the
long bones of the horse and there is little spongy bone.

THE SMALL METACARPAL BONES

These are situated on either side of the posterior surface of the large metacarpal bone, and form the sides of the metacarpal groove. Each consists of a shaft and two extremities.

The =shaft= (Corpus) is three-sided and tapers to the distal end. It is variably curved, convex toward the middle line of the limb. The anterior surface is flattened and is rough, except in its lower part; it is attached to the large metacarpal bone by an interosseous ligament, except near the distal end. The abaxial surface is smooth and rounded from side to side above, grooved below. The axial surface is smooth and concave from edge to edge, except below, where it forms a rounded edge.

The =proximal extremity= or =head= (Basis) is relatively large. In the case of the inner bone it usually bears two facets above which support the second and third carpal bones, while the outer bone has here a single facet for articulation with the fourth carpal bone. Each has also two facets for articulation with the large metacarpal, and is elsewhere roughened for the attachment of ligaments and muscles. The inner bone may present a small facet behind for the first carpal bone.

The =distal extremity= (Capitulum) is usually a small nodule, which projects to a variable extent in different subjects, and is easily felt in the living animal. It is situated two-thirds to three-fourths of the way down the region.

The small metacarpal bones vary much in length, thickness, and
curvature. In the majority of cases the inner bone is the longer; in
other subjects the outer one is the longer or there is no material
difference. Sometimes the curvature is very pronounced, so that the
distal end causes a decided projection. The distal end is very
variable in size and may be a mere point.

=Development.=—The large metacarpal bone ossifies from three centers. The proximal extremity unites with the shaft before birth, the distal extremity toward the middle of the second year. The small metacarpal bones ossify from two centers, one of which is for the proximal extremity. Their distal ends are cartilaginous at birth. Fusion of the middle part of the shaft with the large metacarpal bone is common.

THE PHALANGES

THE FIRST PHALANX

The _first phalanx_ (Phalanx prima)[22] is a long bone, situated between the large metacarpal bone above and the second phalanx below. It is directed obliquely downward and forward, forming an angle of 50 to 55 degrees with the horizontal plane in well-formed limbs. It consists of a shaft and two extremities.

The =shaft= (Corpus) is wider and much thicker above than below, and presents two surfaces and two borders. The =anterior= or =dorsal surface= is convex from side to side and smooth. The =posterior= or =volar surface= is flattened, and bears a triangular rough area, bounded laterally by ridges which begin at the tuberosities above and converge below; this area furnishes attachment to the inferior sesamoidean ligaments. The =borders=, =internal= and =external=, are rounded and have a rough area or a tubercle on their middle parts.

The =proximal extremity= (Basis) is relatively large. It bears an articular surface adapted to the distal end of the large metacarpal bone, consisting of two glenoid cavities separated by a sagittal groove; the inner cavity is a little larger than the outer one. The posterior angles are formed by buttress-like tuberosities for ligamentous attachment. The anterior surface has a slight elevation for the attachment of the lateral extensor tendon.

The =distal extremity= is smaller, especially in its antero-posterior diameter. It presents a trochlea for articulation with the second phalanx, consisting of a shallow central groove and two lateral convex areas or condyles; the inner area is a little the larger. On either side, just above the margin of the articular surface, is a depression surmounted by a tubercle, to both of which the lateral ligament is attached. Behind the tubercle is a distinct facet to which the superficial flexor tendon is attached.

=Development.= The first phalanx ossifies from three centers. The distal end unites with the shaft before birth, the proximal end early in the first year.

The first phalanx contains a small medullary canal in the middle of
the shaft. It may be remarked that the bone is twisted slightly; when
placed volar surface down on the table, it touches the latter by three
points only, the proximal tuberosities and the internal condyle.

THE SECOND PHALANX

The =second phalanx= (Phalanx secunda)[23] is situated between the first and third phalanges, its direction corresponding to that of the first phalanx. It is flattened from before backward, and its width is greater than its height. It may be described as possessing four surfaces.

The =upper= or =proximal surface= presents two glenoid cavities separated by a low ridge, and articulates with the first phalanx. The middle of the anterior border is elevated and roughened in front for the attachment of the anterior or common extensor tendon. The posterior border is thick and overhanging; in the fresh state its middle part is covered with cartilage, over which the deep flexor tendon passes. On either side there is an eminence, to which the lateral ligament and the superficial flexor tendon are attached.

The =inferior= or =distal surface= is trochlear, and articulates with the third phalanx and third sesamoid bone. It resembles somewhat the trochlea of the first phalanx, but is more extensive and encroaches more on the anterior and posterior surfaces.

The =anterior= or =dorsal surface= is convex from side to side and smooth in its middle; on its lower part are lateral rough depressions, surmounted by tuberosities, to both of which ligaments are attached.

The =posterior= or =volar= surface is smooth, flattened, and slopes obliquely downward and forward. The borders which separate the anterior and posterior surfaces are concave from above downward, rounded from before backward.

=Development.=—The second phalanx ossifies like the first, but the proximal end unites with the shaft two or three months earlier.

FIG. 58.—PHALANGES AND THIRD SESAMOID OF HORSE, DORSAL ASPECT.
]

THE THIRD PHALANX

The =third= or =ungual phalanx= (Phalanx tertia)[24] is entirely inclosed by the hoof, to which it conforms in a general way. It presents for examination three surfaces, three borders, and two angles or wings.

The =articular surface= (Facies articularis) faces upward and backward, and is chiefly adapted to the distal surface of the second phalanx, but a narrow flattened area along the posterior border articulates with the third sesamoid. The =anterior= or =coronary border= forms a central eminence, the =extensor= (or pyramidal) =process= (Processus extensorius), to the front of which the anterior extensor tendon is attached. On either side is a depression for the attachment of the lateral ligament.

The =dorsal= or =wall surface= (Facies dorsalis) slopes downward and forward. The angle of inclination on the ground plane is about 45 to 50 degrees in front. Laterally the height diminishes, and the slope becomes steeper, especially on the inner side. From side to side the curvature is almost semicircular. The surface is rough and porous, resembling pumice stone somewhat. It is perforated by numerous foramina of various sizes; a series of larger ones is situated on or near the lower border. On either side the =dorsal= (or preplantar) =groove= (Sulcus dorsalis) passes forward from the wing and ends at one of the larger foramina. In the fresh state this surface is covered by the matrix of the wall of the hoof. The =inferior= or =distal border= is thin, sharp, and irregularly notched; there is commonly a wider notch in front.

The =volar= or =inferior surface= (Facies volaris) is arched, and divided into two unequal parts by a curved rough line, the =semilunar crest= (Crista semilunaris). The larger anterior area is crescent-shaped, concave, and comparatively smooth; it corresponds to the sole of the hoof, and may be termed the =sole surface=. The posterior part is much smaller, and is semilunar; it is related to the deep flexor tendon, and is hence called the =flexor= or =tendon surface= (Facies flexoria). It presents a central prominent rough area, on either side of which is the =volar= (or plantar) =foramen= (Foramen volare), to which the =volar= (or plantar) =groove= (Sulcus volaris) conducts from the wing. The foramina lead into the =semilunar canal= within the bone, from which small canals lead to some of the foramina of the wall surface. The deep flexor tendon is inserted into the semilunar crest and the central rough area behind it.

The volar grooves and foramina transmit the terminations of the
digital arteries into the semilunar canal, where they meet and form a
terminal arch, from which branches pass through canals in the bone and
emerge through the foramina on the wall surface.

The =angles= or =wings= (Anguli) are prismatic masses which project backward on either side; the inner one is usually the shorter. Each is divided into upper and lower parts by a notch, or is perforated by a foramen which leads to the vascular groove on the side of the wall surface.[25] The upper border carries the lateral cartilage.

The =lateral cartilages= (Cartilagines ungulæ) are rhomboid curved plates, which surmount the wings on either side. They are relatively large and extend above the margin of the hoof sufficiently to be distinctly palpable. The abaxial surface is convex, the axial concave. The upper border is convex and thin; the lower is thicker and is in part attached to the wing. The anterior end is attached by ligament to the side of the second phalanx. The posterior end curves toward its fellow at the heel, and is perforated by numerous foramina for the passage of veins. The central part is mainly hyaline, the periphery mainly fibrous.

It will be noted that the size and form of the wings vary much in
different specimens. In the new-born foal the wing is a small, pointed
projection. Later the process of ossification invades the lower part
of the cartilage to a varying extent. In some cases the greater part
of the cartilage is ossified—a condition commonly termed “sidebone.”

=Development.=—The ossification of the terminal phalanx is peculiar. While the proximal articular part is still cartilaginous, a perichondrial cap of bone is formed in relation to the hoof. Later the process extends into the upper part.

=Structure.=—The interior of this bone is channeled by numerous canals for vessels, most of which radiate from the semilunar canal to the wall surface; these are not canals for nutrient vessels of the bone, but transmit arteries to the matrix of the hoof. Thick layers of compact substance are found at the articular and flexor surfaces and the extensor process, _i. e._, at the points of greatest pressure and traction.

THE SESAMOID BONES

The two =proximal= or =great sesamoids= (Ossa sesamoidea phalangis primæ) are situated behind the distal end of the large metacarpal bone, and are closely attached to the first phalanx by strong ligaments. Each has the form of a three-sided pyramid. The =anterior= or =articular surface= conforms to the corresponding part of the distal end of the large metacarpal bone. The =posterior= or =flexor surface= is flattened and oblique; in the fresh state it is covered by a layer of cartilage which also fills the interval between the opposed borders of the two bones, and forms a smooth groove for the deep flexor tendon. The =abaxial surface= is concave, and gives attachment to part of the suspensory ligament; it is separated from the posterior surface by a rough everted border. The =base= faces downward, and furnishes attachment to the inferior sesamoidean ligaments. The =apex= is directed upward and is rounded.

The =third sesamoid= or =navicular= bone (Os sesamoideum phalangis tertiæ) is shuttle-shaped, and is situated behind the junction of the second and third phalanges. Its long axis is transverse, and it possesses two surfaces, two borders, and two extremities. The =articular surface= (Facies articularis) faces upward and forward; it consists of a central eminence, flanked by concave areas, and articulates with the distal end of the second phalanx. The =flexor= or =tendon surface= (Facies flexoria) is directed downward and backward. It resembles the articular surface in form, but is more extensive and not so smooth. In the fresh state it is coated with cartilage and the deep flexor tendon plays over it. The =proximal border= (Margo liber) is wide and grooved in its middle, narrower and rounded on either side. The =distal border= (Margo ligamenti) bears in front a narrow facet for articulation with the third phalanx. Behind this is a groove, which contains a number of relatively large foramina, and is bounded behind by a prominent edge. The =extremities= are blunt-pointed.

FIG. 59.—THIRD SESAMOID OR NAVICULAR BONE OF HORSE. (After Schmaltz,
Atlas d. Anat. d. Pferdes.)
]

=Development.=—It ossifies from a single center.

THE BONES OF THE PELVIC LIMB

The =pelvic girdle= consists of the ossa coxæ, which unite ventrally at the symphysis pelvis, and articulate with the sacrum dorsally.

OS COXÆ

The =os coxæ= (or os innominatum) forms the skeleton of the hip or haunch, and is the largest of the flat bones. It consists primarily of three parts, the =ilium=, =ischium=, and =pubis=, which meet to form the =acetabulum=, a large cotyloid cavity for articulation with the head of the femur. These parts are fused at about one year of age, but it is convenient to describe them separately.

THE ILIUM

The =ilium= (Os ilium) is the largest of the three parts. It is irregularly triangular and presents two surfaces, three borders, and three angles.

The =gluteal surface= (Facies glutæa) faces upward, backward, and outward. It is wide and concave in front, narrower and convex behind. The wide part is crossed by the curved =gluteal line= (Linea glutæa), which extends from the middle of the inner border toward the external angle. This surface gives attachment to the middle and deep gluteal muscles.

FIG. 60.—OSSA COXARUM OF MARE, DORSAL VIEW.

_O.il._, Ilium; _O.p._, pubis; _O.is._, ischium; _A.o.i._, wing of
ilium; _C.o.i._, shaft of ilium; _Cr.o.i._, anterior border (crest)
of ilium; _1_, obturator foramen; _2_, acetabulum; _3_, internal
angle of ilium; _4_, external angle of ilium; _5_, gluteal line;
_6_, psoas tubercle; _7_, _8_, acetabular and symphyseal branches of
pubis; _9_, _10_, acetabular and symphyseal branches of ischium;
_12_, lesser sciatic notch; _13_, ischial arch; _14_, great sciatic
notch; _15_, symphysis pelvis; _17_, ilio-pectineal eminence; _18_,
anterior borders of pubic bones; _19_, posterior gluteal line.
(Struska, Anat. d. Haustiere.)
]