Part 11

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.

FIG. 24.—STERNUM OF HORSE, LATERAL VIEW.

The sternebræ are designated by Roman numerals and the costal facets by ordinary figures. ]

The =dorso-lateral= borders separate the dorsal and lateral surfaces. They give attachment to the lateral branches of the sternal ligament.

The =ventral border= forms the prominent keel-like crest of the sternum (Crista sterni) which may be felt in the living animal; it fades out behind.

FIG. 25.—ANTERIOR APERTURE OF THORAX OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =anterior extremity= or =manubrium sterni= can be distinctly felt in the central furrow of the breast. It consists of a laterally compressed cartilaginous prolongation, commonly called the =cariniform cartilage=. Its lateral surfaces are flat and furnish attachment to muscles of the breast and neck. The ventral border is rounded, and is continued backward on the body of the bone. The dorsal border is concave and has an articular cavity for the first pair of costal cartilages.