Part 4

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.

[Illustration:

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.

[Illustration:

FIG. 20.—FIRST COCCYGEAL VERTEBRA OF HORSE, LEFT VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

[Illustration:

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.

[Illustration:

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

[Illustration:

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.

[Illustration:

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.

[Illustration:

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

The =anterior extremity= or =manubrium sterni=[5] 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.

The =posterior extremity= is formed by the =xiphoid= or =ensiform cartilage= (Processus xiphoideus). This is a thin plate, connected in front with the last bony segment by a relatively thick, narrow neck, and expanding in nearly circular form behind and laterally. Its dorsal surface is concave, and gives attachment to the diaphragm. The ventral surface is convex. The free margin is very thin.

=Development.=—At birth the sternum of the horse consists of seven bony segments or sternebræ united by intersternebral cartilages. The last two sternebræ fuse in the second month, but the others do not usually unite completely even in old age. The sternebræ consist of very vascular spongy bone covered by a very thin layer of compact substance. The adult sternum thus consists to a very considerable extent of persisting cartilage, viz., the intersternebral cartilages, the ventral keel, and the extremities; in old age these undergo partial ossification.

THE THORAX

The bony thorax of the horse is remarkably compressed laterally in its anterior part, but widens greatly behind. The =anterior aperture= is oval and very narrow below; in a horse of medium size its greatest width is about 4 inches (10 cm.), and its height 7 to 8 inches (ca. 18 to 20 cm.). The =ventral wall= or =floor= is about 10 inches (40 cm.) long, and the =dorsal wall= or =roof= about 38 to 40 inches (95 to 100 cm.) long. The height from the last segment of the sternum to the seventh or eighth thoracic vertebra is about twice that of the anterior aperture; this is due to the obliquity and divergence of the roof and floor. The greatest width of the posterior aperture is about 20 to 24 inches (50 to 60 cm.). The intercostal spaces increase in width from the first to the seventh or eighth, and then diminish. Their average width is about 1¼ to 1½ inches (3 to 3.5 cm.).

THE BONES OF THE SKULL

(A) BONES OF THE CRANIUM

The bones of the cranium (Ossa cranii) are the Occipital, Sphenoid, Ethmoid, Interparietal, Parietal, Frontal, and Temporal. The first four are single, the others paired.

[Illustration:

FIG. 26.—SKULL, ATLAS, AND AXIS OF HORSE, LATERAL VIEW.

8, Body of mandible; 28′, horizontal (molar) part of ramus; 30, vertical part of ramus; 9, zygomatic process of squamous temporal; 11, coronoid process of mandible; 12, supraorbital process; 13″, paramastoid (styloid) process of occipital; 19, orbit; 20, malar bone; 21, lacrimal bone; 22, nasal bone; 23, premaxilla; 23′, nasal process of premaxilla; 25, 29, canine teeth; 26, maxilla; 27, facial crest; 31, condyle of mandible; 32, atlas; 33, axis; x, wing of atlas; e, naso-maxillary notch. (After Ellenberger-Baum, Anat. für Künstler.) ]

THE OCCIPITAL BONE

The =occipital bone= (Os occipitale) is situated at the posterior part of the cranium, of which it forms the posterior wall and part of the ventral wall or base.[6]

Its lower part is perforated centrally by a large, almost circular opening, the =foramen magnum= (Foramen occipitale magnum), at which the brain and spinal cord join. The foramen is bounded laterally and dorsally by the =lateral parts= of the bone, and ventrally by the =basilar part= or =process=. Above the lateral parts—but not entering into the formation of the foramen magnum—is the =squamous part=.

[Illustration:

FIG. 27.—LINE DRAWING OF POSTERIOR HALF OF BASE OF SKULL OF HORSE, WITHOUT MANDIBLE. (Key to Fig. 28.)

1, Incisura carotica; 2, incisura ovalis; 3, incisura spinosa; 4, external orifice of parieto-temporal canal; 5, Eustachian canal; 6, petro-tympanic fissure; 7, external auditory canal; 8, hyoid process; 9, Vidian groove; 10, supraorbital process; _A_, basilar part of occipital; _B_, body of sphenoid; _C_, temporal wing of sphenoid; _D_, squamous temporal bone; _E_, petrous temporal bone; _F_, orbital part of frontal bone. ]

The =lateral parts= (Partes laterales) bear the =occipital condyles= (Condyli occipitales), which articulate with the atlas. The condyles are obliquely placed, wide apart dorsally, and separated by a small interval ventrally. The articular surface is curved so sharply in the dorso-ventral direction as to form a blunt ridge externally. The cranial surface is concave and smooth. External to the condyle is the =paramastoid= or =styloid process= (Processus jugularis), a strong flattened bar of bone which projects downward and backward; its external surface is convex and roughened for muscular attachment. Between the root of this process and the condyle is a smooth depression, the =condyloid fossa= (Fossa condyloidea inferior); in this is the =hypoglossal foramen= (Foramen hypoglossi), which transmits the nerve of like name.