Part 4

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

=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.