Part 3

The fossil remains of the ancestors of the existing Equidæ illustrate in a most complete manner the reduction which has occurred in this respect. The earliest known ancestor of the horse, Eohippus or Hyracotherium of the Lower Eocene, had four well developed metacarpal bones, each of which carried a digit; the first metacarpal bone was small. Eleven intermediate stages show the gradual evolution of the race from this primitive animal, which was about the size of the domestic cat. There is reason to believe that earlier forms had five digits.

The =digits= (Digiti manus) are homologous with the fingers of man, and are typically five in number. They are designated numerically from the radial to the ulnar side, in correspondence with the metacarpus. The full number is present in the dog. In the ox and pig the third and fourth are well developed and support the weight, while the second and fifth are reduced. The existing horse has a single digit, the third of his polydactyl ancestors. The skeleton of each fully developed digit consists of three =phalanges= and certain =sesamoid bones=. The =first= or =proximal phalanx= (Phalanx prima) articulates with the corresponding metacarpal bone above and with the =second= or =middle phalanx= (Phalanx secunda) below. The =third= or =distal phalanx= (Phalanx tertia) is inclosed in the hoof or claw, and is modified to conform to the latter. The =sesamoid bones= (Ossa sesamoidea) are developed along the course of the flexor tendons or in the joint capsules. Two =proximal sesamoids= (Ossa sesamoidea phalangis primæ) occur at the flexor side of the metacarpo-phalangeal joint and form a pulley for the flexor tendon. The =distal sesamoids= (Ossa sesamoidea phalangis tertiæ) are similarly placed between the deep flexor tendon and the joint between the second and third phalanx; they are absent in the dog, which has a small sesamoid on the extensor side of the metacarpo-phalangeal joints, and often at the proximal interphalangeal joint also.

Numerous cases are recorded of the occurrence of supernumerary digits (hyperdactylism) in the horse and other animals. In some pigs, on the other hand, the two chief digits are fused, and the condition appears to be inherited.

THE BONES OF THE PELVIC LIMB

The pelvic limb, like the thoracic, consists of four segments, viz., the pelvic girdle, thigh, leg, and the hind foot or pes; the last is subdivided into tarsus, metatarsus, and digits.

The =pelvic girdle= (Cingulum extremitatis pelvinæ) consists of the =os coxæ= or hip bone, which joins its fellow of the opposite side ventrally at the symphysis pelvis, and articulates very firmly with the sacrum dorsally. The two coxal bones, together with the sacrum and the first two or three coccygeal vertebræ, constitute the bony pelvis. The os coxæ consists originally of three flat bones, the =ilium=, =ischium=, and =pubis=, which meet at the acetabulum, a large cotyloid cavity that articulates with the head of the femur. These three parts are fused before growth is complete, but are considered separately for convenience of description. The =ilium= (Os ilium) is situated in the lateral wall of the pelvis, the =pubis= (Os pubis) in the anterior part, and the =ischium= (Os ischii) in the posterior part of the ventral wall.

The =thigh= (Femur), like the arm, contains a single large, long bone, the =femur= or thigh bone (Os femoris). This articulates with the acetabulum above and the tibia and patella below.

The skeleton of the leg (Crus) comprises three bones (Ossa cruris), viz., the =tibia=, =fibula=, and =patella=. The =tibia= is a large prismatic long bone which supports the weight, and articulates below with the tibial tarsal bone. The =fibula= is situated along the outer border of the tibia, from which it is separated by the =interosseous space= of the leg. It is much more slender than the tibia and does not articulate with the femur. In the pig and dog it has a complete shaft and two extremities, but in the horse and ox it is much reduced and otherwise modified. The =patella= or “knee-cap” is a short bone which articulates with the trochlea of the distal end of the femur; it is to be regarded as a large sesamoid bone intercalated in the tendon of the quadriceps femoris muscle.

The =tarsus= or “hock” consists of a group of short bones (Ossa tarsi) numbering five to seven in the different animals. The proximal or crural row consists of two bones, the =tibial= and =fibular tarsals=; the former is situated at the inner or tibial side, and has a trochlea for articulation with the distal end of the tibia; the latter, situated externally, has a process, the =tuber calcis=, which projects upward and backward and constitutes a lever for the muscles which extend the hock joint. The distal or metatarsal row consists of four bones when seven tarsal elements are present, as in the pig and dog. They are best designated numerically as =first tarsal=, =second tarsal=, etc. The =central tarsal= is interposed between the rows.

The preceding terms are anglicized abbreviations of those introduced by Gegenbaur into comparative anatomy. The Latin names and synonyms are given in the following table.

Tibial (Os tarsi tibiale, Tt.) Astragalus or Talus Fibular (Os tarsi fibulare, Tf.) Calcaneus or Os calcis Central (Os tarsi centrale, Tc.) Scaphoid or Navicular First Tarsal (Os tarsale primum, T1) First or internal cuneiform Second Tarsal (Os tarsale secundum, T2) Second or middle cuneiform Third Tarsal (Os tarsale tertium, T3) Third or external cuneiform Fourth Tarsal (Os tarsale quartum, T4) Cuboid.

The =metatarsal= and =digital bones= resemble in general those of the corresponding regions of the thoracic limb; the differential features will be noted in the special descriptions.

[Illustration:

FIG. 5.—SKELETON OF HORSE, WITH OUTLINE OF CONTOUR OF BODY.

_1.H._, Atlas; _7.H._, seventh cervical vertebra; _1.R._, first thoracic vertebra; _17.R._, seventeenth thoracic vertebra; _1.L._, first lumbar vertebra; _6.L._, sixth lumbar vertebra; _K_, sacrum; _1.S._, first coccygeal vertebra; _16.S._, sixteenth coccygeal vertebra; _6.R._, sixth rib; _6.K._, costal cartilage; _18.R._, last rib; 1, scapula; 1′, cartilage of scapula; 2, spine of scapula; 4, humerus; 4′ external epicondyle of humerus; 5, external tuberosity of humerus; 6, deltoid tuberosity; 7, shaft of ulna; 8, olecranon; 9, radius; 10, carpus; 11, accessory carpal bone; 12, metacarpus; 13, digit; 14, sternum; 14″, xiphoid cartilage; 15, ilium; 16, 16′, external and internal angles of ilium; 17, ischium; 18, femur (shaft); 19, trochanter major; 27, trochanter minor; 28, trochanter tertius; 20, patella; 21, tibia (shaft); 21′, external condyle of tibia; 23, fibula; 22, tarsus; 24, tuber calcis; 25, metatarsus; 26, digit. (After Ellenberger-Baum, Anat. für Künstler.) ]

THE SKELETON OF THE HORSE

The skeleton of the horse consists of 205 bones, as shown in the following table.

Vertebral column 54 Ribs 36 Sternum 1 Skull (including auditory ossicles) 34 Thoracic limbs 40 Pelvic limbs 40 ———— 205

In this enumeration the average number of coccygeal vertebræ is taken to be 18, the temporal and os coxæ are not divided into parts, the usual number of carpal and tarsal elements is taken, and the sesamoids are included.

THE VERTEBRAL COLUMN

The vertebral formula of the horse is C_{7}T_{18}L_{6}S_{5}Cy_{15–21}.

[Illustration:

FIG. 6.—CERVICAL VERTEBRÆ OF HORSE, DORSAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

[Illustration:

FIG. 7.—CERVICAL VERTEBRÆ OF HORSE, VENTRAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE CERVICAL VERTEBRÆ

These are quadrangular, massive, and longer than the vertebræ of other regions; they decrease in length from the second to the last. The =third=, =fourth=, and =fifth= are typical, and have the following characters:

1. The =bodies= are long as compared with those of other vertebræ. Each presents a median =ventral spine= or =crest=, which becomes more prominent as it is traced backward, and is tuberculate at its posterior end. The =lateral aspect= is concave. The =dorsal surface= has a flat central area which is narrow in the middle of the vertebræ, and wide at either end; it gives attachment to the superior common ligament. On either side of this area is a groove which lodges the longitudinal spinal vein. These lateral grooves are connected at the middle of the surface by a transverse furrow, in which there are several foramina through which veins emerge from the spongy substance of the body. The =anterior extremity= presents a head which has an oval articular surface, strongly convex, and wider above than below. The =posterior extremity= is larger and has a nearly circular cotyloid cavity.

[Illustration:

FIG. 8.—LAST CERVICAL AND FIRST THORACIC VERTEBRÆ OF HORSE, LATERAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

2. The =arches= are large and strong. They are perforated on either side by a foramen which communicates with the foramen transversarium. The vertebral notches are large.

3. The =articular processes= are large. Their articular surfaces are extensive, oval in outline, and slightly concave; the anterior ones are directed upward and inward, the posterior downward and outward. The remaining surface is mainly roughened for ligamentous and muscular attachment. A crest connects the articular processes of the same side on the fourth and fifth; on the third it does not reach the anterior process.

4. The =transverse processes= are large and plate-like. Each arises by two roots, one from the arch and one from the body; between these is the =foramen transversarium=, through which the vertebral artery passes. The process divides externally into anterior and posterior branches, which are thickened and rough for muscular attachment.

5. The =spinous process= is represented by a crest (Crista spinosa), which widens behind, and is connected by ridges with the posterior articular processes.

The =sixth= cervical vertebra has the following distinctive features: It is shorter and wider than the fifth. The =arch= is large, especially posteriorly. The =posterior articular= processes are shorter, thicker and further apart; they are connected with the anterior ones by a thick ridge. The =spinous process= is less rudimentary; it is half an inch or more (ca. 1.5 cm.) in height. The =transverse processes= have three branches; the third part is a thick, almost sagittal plate, which forms with its fellow and the body a wide ventral groove on the posterior part of the vertebra; the other branches correspond to those of the typical vertebræ, but are short and thicker. The =foramen transversarium= is large; below its posterior end is a fossa. The =ventral crest= is small and is less prominent posteriorly.

The third branch of the transverse process and the fossa are sometimes absent on one side.

The =seventh= cervical vertebra is readily distinguished by the following characters: It is =shorter= and =wider= than the others. The =body= is flattened dorso-ventrally and wide, especially behind; here it has a =demifacet= on either side for articulation with part of the head of the first rib. The =arch= and its =notches= are large. The =anterior articular processes= are wider and longer than the posterior pair. The =spinous process= is an inch or more (ca. 3 cm.) in height. The =transverse process= is undivided, and has no foramen transversarium. The ventral crest is replaced by a pair of tubercles.

In some specimens a large foramen transversarium is present on one side or (rarely) on both.

[Illustration:

FIG. 9.—ATLAS OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

[Illustration:

FIG. 10.—ATLAS OF HORSE, POSTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE ATLAS

This vertebra is decidedly atypical in form and structure. The body and spinous process are absent. It has the form of a strong ring, from which two curved plates, the =wings=, project laterally. The ring incloses a very large vertebral foramen, and consists of two =lateral masses= connected by =dorsal= and =ventral arches=.

The =lateral masses= (Massæ laterales) present two deep oval =anterior articular cavities= (Foveæ articulares craniales) which receive the occipital condyles; they are separated by a wide notch above and a narrow one below. The outer margin is also notched, and a triangular non-articular depression cuts into the inner part of each cavity. The =posterior articular surfaces= (Facies articulares caudales) are somewhat saddle-shaped; they are confluent on the ventral arch below, but are widely separated above, and do not conform in shape to the corresponding surfaces of the axis.

The =dorsal arch= (Arcus dorsalis) presents a median =dorsal tubercle= (Tuberculum dorsale) and is concave below. It is perforated on either side near its anterior margin by the =intervertebral foramen=. The =anterior border= is deeply notched, and the =posterior= is thin and concave.

The =ventral arch= (Arcus ventralis) is thicker, narrower, and less curved than the dorsal. On its lower surface is the =ventral tubercle= (Tuberculum ventrale), into which the terminal tendon of the longus colli muscle is inserted. The upper face has posteriorly a transversely concave articular surface (Fovea dentis), on which the dens or odontoid process of the axis rests. In front of this is a transverse rough excavation for the attachment of the odontoid ligament.

The =wings= (Alæ) are modified transverse processes. They are extensive curved plates which project outward, downward, and backward from the lateral masses. The dorsal surface is concave. Between the ventral aspect of the wing and the lateral mass is a cavity, the =fossa atlantis=; in this there is a foramen which opens into the vertebral canal. The =border= is thick and rough. Two foramina perforate each wing. The anterior one, the =foramen alare=, is connected with the intervertebral foramen by a short groove. The posterior one is the =foramen transversarium=.

=Development.=—The atlas ossifies from =three= or =four centers=, one or two for the ventral arch, and one on either side for each lateral mass, wing, and half of the dorsal arch. At birth the bone consists of three pieces—the ventral arch and two lateral parts, which are separated by a layer of cartilage in the dorsal median line. These parts are usually fused at about six months.

[Illustration:

FIG. 11.—FIRST THREE CERVICAL VERTEBRÆ OF HORSE, LATERAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE AXIS

The =axis= (Epistropheus) is the longest of the vertebræ, and is characterized by the presence of the odontoid process, which projects from the anterior part of the body.

The =body= has a median ventral crest which terminates in a tubercle behind. The anterior extremity presents centrally the =odontoid process= (Dens); this has a convex articular surface ventrally for articulation with the ventral arch of the atlas, and two rough depressions for the attachment of the odontoid ligament dorsally. Flanking this on either side are the modified =anterior articular processes=, which have saddle-shaped articular surfaces confluent below with that of the dens. The posterior extremity has the usual cavity.

The =arch= presents in the young subject a notch on each side of its anterior border; this is converted into a foramen by a ligament which usually ossifies later. The posterior border has the usual notches.

The =posterior articular processes= are typical.

The =transverse processes= are small, single, and project backward. The =foramen transversarium= is small.

The =spinous process= is very large and strong. Its free border is rough, thickens posteriorly, and is continued to the articular processes by two ridges. The lateral surfaces are concave and rough for muscular attachment.

=Development.=—The axis has =six= or =seven centers= of ossification. In addition to the usual five, one or two appear for the dens, which is regarded as the displaced body of the atlas. A nucleus behind the dens, which remains distinct to three or four years of age, is considered by Lesbre to be the head of the axis.

[Illustration:

FIG. 12.—OCCIPITAL BONE AND FIRST THREE CERVICAL VERTEBRÆ OF HORSE, DORSAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE THORACIC VERTEBRÆ

These (Vertebræ thoracales) are usually eighteen in number in the horse, but there are sometimes nineteen, rarely seventeen. As regional characters we note the surfaces for articulation with the ribs and the length and form of the spinous processes. Those in the middle of the series are the most typical and present the following features:

1. The =bodies= are short and constricted in the middle. The ends are expanded and have articular surfaces which are not strongly curved. On the upper part of each side are anterior and posterior =costal facets= (Fovea costalis cranialis, caudalis), which, with those of adjacent vertebræ, form sockets for the heads of the ribs.

2. The =arches= are small. Their posterior notches are relatively large and are often converted into foramina.

[Illustration:

FIG. 13.—THIRD, FOURTH, AND FIFTH THORACIC VERTEBRÆ OF HORSE, LATERAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

3. The =articular processes= are small. The anterior pair are in fact represented only by two oval facets on the anterior part of the laminæ which face almost directly upward. The posterior pair spring from the base of the spinous process; their facets face almost directly downward.

4. The =transverse processes= are short, thick, and tuberous at the free end. Each has a =facet= (Fovea transversaria) for articulation with the tuberosity of the rib which has the same serial number.

5. The =spinous process= is large, narrow, and slopes upward and backward. The anterior border is thin, the posterior wider and furrowed. The summit is expanded and rough.

The =first thoracic vertebra= is easily recognized by the following specific characters: The body is wide and flattened dorso-ventrally. In front it has a head like the cervical vertebræ, and behind a cavity somewhat deeper than any other thoracic vertebra. Two =costal facets= are found on either side, and a well-marked spine ventrally. The =arch= is large and strong, and has large notches. The =articular processes= are much larger than those of other thoracic vertebræ, and resemble a good deal those of the seventh cervical in form. The =transverse processes= are short and thick, and each has on its ventral aspect a large concave facet for articulation with the tubercle of the first rib. The spinous process is curved backward and tapers to a point. Its length is usually about three or four inches (ca. 8 to 10 cm.). It may be mistaken at first glance for the last cervical, but is promptly identified by the three costal facets on each side and the length of the spine.

The =last thoracic vertebra= is distinguished by the absence of the posterior pair of costal facets, and the confluence of the anterior pair with those on the transverse processes.

[Illustration:

FIG. 14.—THIRD THORACIC VERTEBRÆ AND UPPER PART OF RIB OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

[Illustration:

FIG. 15.—LOWER PART OF NINTH THORACIC VERTEBRA OF HORSE, POSTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The serial position of others may be determined at least approximately by the following data: (1) The bodies gradually diminish in length and width to the middle of the region and then increase slightly. Their costal facets become smaller and less concave from first to last. The ventral crest is distinct on three or four vertebræ at either end of the region. (2) The transverse processes diminish in size and are placed lower down as they are traced backward. Their costal facets become smaller and lower in position; on the last (and sometimes on its predecessor also) it fuses with the costal facet of the body. The upper non-articular part of the process gradually becomes more sharply defined, and in the last four or five separates to form the =mammillary process=. (3) The spinous processes increase in length to the third and fourth, and then gradually diminish to the fifteenth, beyond which they have about the same length. The backward inclination is most pronounced in the second, the sixteenth is vertical, and the last two are directed a little forward. The longest spines (_i. e._, those of the withers) are the thickest and have expanded summits which remain more or less cartilaginous; the others are more plate-like, and are surmounted by a thick lip. The second spine is more than twice as large as the first. The summits of the fourth and fifth usually form the highest point of the withers.

=Development.=—There are =six= or =seven centers=, three for the body, two for the arch, and one for the spinous process; some of the latter have an additional center for the summit.

THE LUMBAR VERTEBRÆ

The =lumbar vertebræ= (Vertebræ lumbales) are usually six in number in the horse. They are characterized by the size and form of their transverse processes.

The =bodies= of the first three are triangular on cross-section, and present a distinct ventral crest. From the fourth backward they become wider and flatter and the ventral crest fades out.

[Illustration:

FIG. 16.—FIRST THORACIC VERTEBRA AND UPPER PART OF FIRST RIB OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =arches= of the first two or three are about equal in size and similar to that of the last thoracic; from the fourth they decrease noticeably in breadth and height.

[Illustration:

FIG. 17.—LAST TWO THORACIC AND FIRST LUMBAR VERTEBRÆ OF HORSE, LATERAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =anterior articular processes= are fused with the mammillary processes, and present superiorly concave surfaces for articulation with the posterior pair of the preceding vertebra. The =posterior articular processes= project distinctly from the arch at the base of the spinous process, and have ventrally convex articular surfaces, which fit into the grooved surfaces of the anterior pair of the next vertebra.

The =transverse processes= are large plates, flattened dorso-ventrally, which project outward and usually curve slightly downward; their length increases to the third and fourth, and then diminishes to the last, which is the shortest. The first one or two usually curve somewhat backward, the last two decidedly forward. Those of the fifth have an oval concave facet on the inner part of the posterior border for articulation with the sixth process; the latter has a corresponding convex facet on the anterior border, and a larger concave surface on the posterior border for articulation with the wing of the sacrum. Sometimes the fifth process has a small surface for articulation with the fourth. The inner part of the sixth process is thick, the outer part thinner, narrower, and curved forward. The inner part of the fifth is also somewhat thickened.

The =spinous processes= resemble those of the last two thoracic vertebræ. They are usually about equal in height, but minor differences are common, and the width diminishes in the last three.

=Development.=—This is similar to that of the thoracic vertebræ. The extremities of the transverse processes remain cartilaginous for some time after ossification is otherwise complete.

[Illustration:

FIG. 18.—SECOND LUMBAR VERTEBRA OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The transverse processes of this region are considered equivalent to the proper transverse process + the costal element, and hence the distinctive term processus lateralis (s. costarius) has been proposed. The occurrence of a lumbar rib in connection with the transverse process of the first lumbar is not rare. Reduction of the number to five has been observed frequently, and may or may not be compensated by an additional thoracic vertebra. This variation is not more common in certain races as Sanson and others have maintained. Very few cases are recorded of seven lumbar vertebræ—especially with the normal thoracic number. An anomalous vertebra with mixed thoracic and lumbar characters sometimes occurs at the junction of the two regions.

THE SACRUM

The =sacrum= (Os sacrum) is formed by the fusion of five vertebræ usually, and is conveniently described as a single bone. It is triangular in form and is wedged in between the ilia, with which it articulates very firmly on each side. Its long axis is gently curved, and slightly oblique, so that the posterior end is a little higher than the anterior. It presents two surfaces, two borders, a base, and an apex.

The =dorsal surface= presents centrally the five sacral spines, which are directed upward and backward, and have (with the exception of the first) tuberous summits which are sometimes bifid.