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 C7T18L6S5Cy15–21.
Fig. 6.—Cervical Vertebræ of Horse, Dorsal View. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
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.
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.
Fig. 9.—Atlas of Horse, Anterior View. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
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.
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.
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.
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.
Fig. 14.—Third Thoracic Vertebræ and Upper Part of Rib of Horse, Anterior View. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
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.
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.
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.
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.
The first spine is relatively thin and narrow, and is not so high as the internal angle of the ilium. The second is the longest, and the height diminishes rapidly to the last. The bases of the spines are often fused in old subjects.
On either side of the spines is a groove, in which are the four dorsal sacral foramina (Foramina sacralia dorsalia); the dorsal branches of the sacral nerves emerge through them.
The ventral or pelvic surface (Facies pelvina) is concave in its length, wide in front, narrow behind. It is marked by four more or less distinct transverse lines (Linæ transversæ), which indicate the demarcation of the bodies of the vertebræ. At the ends of these lines are the ventral sacral foramina (Foramina sacralia ventralia), which are larger than the dorsal series and diminish in size from first to last; they transmit the ventral divisions of the sacral nerves.
The dorsal and ventral foramina communicate with the sacral canal and are together equivalent to the usual intervertebral foramina.
Fig. 19.—Sacrum of Horse, Dorso-lateral View.
C, Body of first sacral vertebra; A, arch of first vertebra; Cs, sacral canal; P.s. 1–5, sacral spines; P.t., wings of sacrum; 1, 1′, surfaces for articulation with transverse processes of last lumbar vertebra; F.a., auricular surface; 2, 2′, articular processes; 3–6, dorsal sacral foramina; 7, interarcuate space; 8, lateral border; h, apex. (Struska, Anat. d. Haustiere.)
The lateral borders are rough, thick in front, thin behind.
The base (Basis ossis sacri) is directed forward, and is relatively very wide. It presents centrally the body of the first sacral segment, which is wide transversely, flattened dorso-ventrally, and has a rounded surface which articulates with the last lumbar vertebra through the medium of an intervertebral fibro-cartilage. The ventral margin projects slightly, forming the promontory (Promontorium). Above the body is the entrance to the sacral canal, flanked by a pair of articular processes, which project upward and forward from the arch, and have concave surfaces internally for articulation with those of the last lumbar vertebra. On each side of these is a smooth notch which is converted into a foramen by apposition with the last lumbar. The lateral parts of the base, the wings or alæ (Alæ sacrales), are strong prismatic masses with pointed ends, which result from the fusion of the first with part of the second transverse process. Each has in front a large, oval, slightly convex surface for articulation with the transverse process of the last lumbar. Posteriorly there is an elongated oval area which faces upward, backward, and outward. This is the auricular surface (Facies auricularis), which articulates with the ilium; it is slightly concave in its length, and somewhat rough and irregular. The rest of the dorsal surface of the wing is roughened for ligamentous attachment, while the ventral surface is smooth.
The apex (Apex ossis sacri) is the posterior aspect of the last sacral vertebra and is quite small. It presents the elliptical flattened surface of the body, above which is the triangular posterior opening of the sacral canal, surmounted by the last sacral spine. There is a pair of narrow notches between the arch and body, above which rudiments of articular processes may occur.
The name sacral canal (Canalis sacralis) is applied to that part of the vertebral canal which traverses the sacrum. Its anterior part is large and has the form of a triangle with the angles rounded off; its width is about twice its height. Traced backward it is seen to diminish in size rapidly, and the posterior opening is quite small and triangular.
The term lateral part (Pars lateralis) designates the portion external to the foramina, which results from the fusion of the transverse processes.
Fig. 20.—First Coccygeal Vertebra of Horse, Left View. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
Fig. 21.—First Coccygeal Vertebra of Horse, Dorsal View. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
Development.—The several sacral vertebræ ossify in the typical manner. Separate centers for costal elements in the lateral parts have not yet been found in the domesticated animals. Fusion begins in front, and is usually not complete behind till near adult age. The lateral parts unite before the bodies. It is rather curious that the epiphyseal plates of adjacent segments unite with each other before they fuse with the main portion of the bodies.
THE COCCYGEAL VERTEBRÆ
These (Vertebræ coccygeæ) vary considerably in number, but eighteen may be taken as an average. From first to last they become reduced in size and, with the exception of a few at the beginning of the series, consist of bodies only. The first three have bodies which are somewhat flattened dorso-ventrally, constricted in the middle, and have at the ends slightly convex, elliptical, articular surfaces. The ventral surface has a median groove for the coccygeal artery. The arch is small and triangular; it is formed of two flat plates which are prolonged to form a short spinous process with a thickened and often double summit. The anterior notches are absent. Functional articular processes are not present, but small rudiments of the anterior pair commonly occur. The transverse processes are relatively large plates which project horizontally outward. Further back the arch becomes incomplete, open above, and soon disappears; the transverse processes gradually fade out, and the vertebræ are reduced to cylindrical rods of diminishing size. The last one has a pointed end.
Variations.—The number is said by good observers to vary between fourteen and twenty-one. In old age the first is often fused with the sacrum, and sometimes with the second. The arch of the third may be open.
THE VERTEBRAL COLUMN AS A WHOLE
In the mid-dorsal line is the series of spinous processes, which are low ridges in the cervical region with the exception of the second and seventh, reach their maximum height at the fourth and fifth thoracic vertebræ, and diminish to the fifteenth or sixteenth thoracic. Behind this they are about equal in height as far as the last lumbar and first sacral, which are somewhat lower. The second sacral spine is about as high as the middle lumbar; behind this they diminish rather rapidly in height and fade out about the third coccygeal. Their inclination backward is most decided at the second thoracic, diminishes from the sixth or seventh to the fifteenth or sixteenth, which is vertical and is termed the anticlinal or diaphragmatic vertebra. Behind this they are inclined a little forward until the sacrum is reached; here there is an abrupt change to the backward inclination, so that a considerable interspinous angle is formed.
On either side of the spinous processes is a vertebral groove which contains the deep muscles of the spine. The floor of the groove is formed by the laminæ and articular processes. It is wide in the neck and narrows progressively in the back.
Viewed from the side, the column presents a series of curves. When the head and neck are in the ordinary neutral position, the anterior part of the cervical spine forms a gentle curve, concave ventrally. The posterior cervical and first thoracic vertebræ form a more pronounced curve in the opposite direction. At the junction of the cervical and thoracic regions there is a marked change of direction, forming a ventral projection or angle. At the second thoracic vertebra a gentle curve, concave ventrally, begins. This is continued to the lumbo-sacral junction, where there is a change of direction, and hence a promontory. The sacrum has a variable, but never very pronounced, ventral concave curvature, which is continued in a much accentuated form in the coccygeal region. It may be remarked that a line through the summits of the spines does not correspond to these curves formed by the bodies.
The vertebral canal, of course, corresponds in curvature to the bodies. Its caliber varies greatly at different points. The greatest diameter (ca. 5 cm.) is in the atlas, where it contains the dens of the axis in addition to the spinal cord, and provision must be made for extensive movement. It is very much smaller in the axis (ca. 2.5 cm. wide, 3 cm. high). It widens considerably at the junction of the cervical and thoracic regions to accommodate the cervical enlargement of the spinal cord. Beyond this it diminishes, and is smaller in the middle of the back than at any preceding point; this is correlated with the small size of the spinal cord and the very limited movement of the spine. At the middle of the lumbar region it again widens considerably to contain the lumbar enlargement of the spinal cord. The caliber diminishes very rapidly from the second sacral segment backward, and the canal ceases to be complete at the fourth coccygeal vertebra.
The articular processes are very large and wide apart in the neck, greatly reduced and much closer together in the back, larger and interlocking in the lumbar region.
The transverse processes are large and outstanding in the neck, where they form the outer boundary of a ventral groove occupied by the longus colli muscle. In the back they are short and stout, and are characterized by the facets for the tubercles of the ribs. On the first thoracic vertebra this facet is large, deeply concave, and situated almost directly outward from the cavity for the head of the rib; traced backward it becomes smaller and flatter, and gradually comes to lie behind the cavity for the head of the rib, with which it is fused on the last and often also on the next to the last thoracic vertebra. The processes in the lumbar region have a characteristic elongated plate-like form. In the sacral region they are fused to form the wings and lateral parts of the sacrum. In the coccygeal region they are at first of considerable size relatively, but undergo rapid reduction, and disappear at the fifth or sixth vertebra.
The cavities for the heads of the ribs diminish progressively in size and depth from first to last.
The mammillary processes are usually distinct on the fourteenth to the seventeenth thoracic vertebræ. In front of these they blend with the transverse, behind with the anterior articular processes.
The length of the vertebral column (including the intervertebral fibro-cartilages) in a horse of medium size is about 260 to 265 cm. (ca. 8 feet 8 inches to 8 feet 10 inches). The relative lengths of the various regions appear to vary most in the neck and back.
The table below gives the measurements in centimeters in a trotting stallion of medium size and in an adult Percheron stallion. The percentages are in round numbers.
| Trotter | Percheron | |||
|---|---|---|---|---|
| Cervical | 60.0 | 22.7% | 74.0 | 25.4% |
| Thoracic | 98.0 | 37.2% | 100.0 | 34.4% |
| Lumbar | 31.5 | 12.0% | 36.5 | 12.5% |
| Sacral | 20.0 | 7.6% | 22.5 | 7.7% |
| Coccygeal | 54.0 | 20.5% | 58.0 | 20.0% |
| 263.5 | 100.0 | 291.0 | 100.0 | |
The Ribs
There are usually eighteen pairs of ribs in the horse, but a nineteenth rib on one side or both is not at all rare. Eight are sternal or “true” ribs, the remainder asternal or “false.” Ribs from different parts of the series vary much in length, curvature, and other characters. We will therefore consider as a type a rib from the middle of the series first, and afterward note the chief serial differences. A typical rib has the following characters:
The shaft or body (Corpus costæ) is elongated, relatively very narrow, and strongly curved; the curvature is most pronounced in the dorsal third, and the ventral part is twisted and inclined inward, so that when a rib is laid with its outer surface on the table, the ventral end is raised. The external surface is convex in its length and also transversely; its anterior part is, however, grooved longitudinally. A distinct angle, i. e., a point at which the curve of the rib changes rather suddenly, as in man, can scarcely be said to exist in the horse. The term is often applied, however, to a corresponding rough elevation which gives attachment to the ilio-costalis muscle; it is most distinct on the fourth to the eighth inclusive. The internal surface is smooth, concave in its length, and rounded from side to side; the costal groove, situated posteriorly, is very distinct above and fades out about the middle. The anterior border is concave, the posterior convex.
The vertebral extremity (Extremitas vertebralis) consists of the head, neck, and tubercle. The head (Capitulum costæ) has an articular surface (Facies articularis capituli costæ), composed of two convex facets, anterior and posterior, separated by a groove for the attachment of the conjugal ligament. It articulates with the cavity formed by facets on the bodies of two adjacent thoracic vertebræ and the intervertebral fibro-cartilage. The neck (Collum costæ) is roughened above and in front. The tubercle (Tuberculum costæ) is placed above and behind the junction of neck and shaft; it has a small surface (Facies articularis tuberculi costæ) for articulation with the transverse process of the corresponding thoracic vertebra.
The sternal extremity (Extremitas sternalis) is somewhat expanded, and is continued by the costal cartilage.
The first rib is easily distinguished. It is the shortest and least curved. At the lower part of the anterior border there is a smooth impression where the brachial vein curves around it; above this is usually the scalene tubercle. The costal groove is absent. The head is large and has two facets of unequal extent which meet at an acute angle in front; the smaller one faces forward and articulates with the last cervical vertebra; the larger one is directed inward and articulates with the first thoracic vertebra. The neck is thick and very short. The tubercle is larger than that of any other rib and has an extensive articular surface which is convex in its length. The sternal end is larger than that of any other rib; it is thick and very wide, and is turned a little forward.
Fig. 22.—Left Seventh Rib of Horse, Antero-external View. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
The last rib is the most slender and regularly curved. It is usually but little longer than the second. The facet on the tubercle is confluent with that of the head. (This feature, however, is usual on the seventeenth also, and not rare on the sixteenth.)
The serial position of the other ribs may be determined approximately by the following considerations: The length increases from the first to the tenth and eleventh and then diminishes. The width increases somewhat to the sixth and then diminishes. The anterior border is thin and sharp from the second to the eighth, and behind this becomes thick and rounded. The external groove is distinct on the fourth to the eighth inclusive. The curvature increases in degree rapidly from the first to the seventh, remains about the same to the sixteenth, and then decreases very noticeably. In regard to dorso-ventral direction, the first rib inclines a little forward, the second is about vertical, while behind this they slope backward in increasing degree, so that a plane tangent to the ventral ends of the last pair cuts the third lumbar vertebra. The head and tubercle diminish in size from first to last. Their relative positions change in that the tubercle of the first rib lies almost directly outward from the head, while further back it gradually comes to lie behind it. The neck is longest on the longest ribs, and is absent on the last two or three. A costo-transverse foramen is formed between the neck and the transverse process.
Development.—The ribs ossify in cartilage from three centers, one each for the shaft, head, and tubercle; the third center is absent in some of the posterior ribs.
Variations.—A nineteenth rib on one side or both is not rare. It is usually imperfectly developed and quite variable. In some cases it is a mere strip of cartilage connected by ligament with the first lumbar transverse process; in other cases it is ossified, and may be fused with the process; in others again it is connected with an additional presacral vertebra which may be thoracic or lumbar or ambiguous in character. It is commonly floating, but may be attached to the eighteenth. Reduction in number is uncommon. Fusion of adjacent ribs sometimes occurs.
THE COSTAL CARTILAGES
Fig. 23.—Sternum and Costal Cartilages of Horse, Ventral View. (After Ellenberger-Baum, Anat. f. Künstler.)
The first costal cartilage is an inch or more (2.5 to 3 cm.) in length. The upper part is very wide and thick. The sternal end is small. The two articulate with each other as well as with the sternum. The cartilages of the other sternal ribs increase progressively in length and become more rounded. The sternal end is expanded and has an elliptical convex facet for articulation with the sternum. The cartilages of the asternal ribs are long, slender, and pointed. The ninth is the longest, and is very firmly attached to the eighth; behind this they diminish progressively in size, and are attached to each other by elastic tissue. Except in the case of the first, the cartilage does not continue the direction of the rib, but forms with the latter an angle which is open in front, and decreases in obtuseness from second to last.
More or less extensive ossification is to be regarded as a normal occurrence, especially in the cartilages of the sternal ribs.
The Sternum
The sternum of the horse is shaped somewhat like a canoe; it is compressed laterally, except in its posterior part, which is flattened dorso-ventrally. It is inclined obliquely so that the posterior end is about six to eight inches (15 to 20 cm.) lower than the anterior.
The dorsal surface has the form of a very narrow isosceles triangle with the apex in front. It is concave longitudinally, flattened transversely.
The lateral surfaces are convex above, slightly concave below, and diminish in extent behind. Each presents on its upper part seven articular cavities (Foveæ costales), which receive the sternal ends of the second to the eighth costal cartilages inclusive. These cavities are situated in series at the intersternebral junctions. The first four are elliptical in outline with the long diameter vertical, and are separated by considerable regular intervals. The others are progressively smaller, more circular, and closer together. The area below these cavities gives attachment to the pectoral muscles.