CONFORMATION AND SURFACE FEATURES

The neck and back vary considerably in conformation according to breed, sex, age, and condition. The dorsal contour of the back and loins closely reflects the course of the vertebral column, but that of the neck, where the vertebrae are more deeply buried, depends largely on the nuchal ligament and crest (see further on).

The neck may be arched, straight, or hollowed in the natural standing posture. The arched form, known to horsemen as a swan- or peacock-neck, is characteristic of certain breeds, including the Lipizzaner. The concave form or ewe-neck is not prized, and for most breeds it is the straight neck that is held in greatest esteem. The transition between the neck and withers may be smooth or marked by a dip. In saddle horses the neck deepens considerably toward the chest, but the change is usually less marked in the heavier draft breeds. Viewed from above, the neck is relatively narrow and of even width, except immediately before the shoulder where the mergence with the trunk is eased by the presence of the subclavius, which fills out the hollow along the cranial margin of the scapula. The heavy neck of the stallion is mainly due to the strong development of the fatty fibrous tissue (crest) dorsal to the nuchal ligament (see Figure 18–38/3).

The course of the cervical vertebrae may not be evident on simple scrutiny, although the wing of the atlas is almost always a prominent visible and palpable landmark. The positions of the transverse and articular processes of the third to sixth neck vertebrae may be visible in animals that are lean or in poor condition. These features are usually detectable on palpation, although in fat or particularly well-muscled horses, it may be impossible to gain more than a general impression of the course of the vertebrae (Figure 19–1). In thin-skinned horses certain of the superficial muscles (especially the trapezius and rhomboideus) stand out as individual surface features when tensed (Figure 19–2/1,8).

The characteristic prominence of the withers is due to the great length of the spinous processes of the second to ninth thoracic vertebrae, but the region also embraces the scapular cartilages and associated muscles. The withers vary considerably, and in saddle animals it is preferred that they be both high and long and of moderate width; excessive narrowness may make a proper fit of the saddle difficult.

Behind the withers the line of the back is more or less straight, and though it slopes up somewhat toward the croup, this is only occasionally so exaggerated that the horse can be said to be “croup high.” There is, however, a tendency for the back to sag in older animals, in those in poor condition, and in mares advanced in pregnancy. The cranial part of the back merges smoothly with the lateral chest and abdominal wall.

The caudal part (the loins) tends to be broader and flatter and merges with the flanks without the sharp change in contour that is so striking in ruminants. The transverse processes of the lumbar vertebrae are not palpable. The spinous processes of the lumbar and caudal thoracic vertebrae may be palpated, though rarely so easily that they can be separately identified and counted. A median groove between the muscles of the loins and croup is most marked in draft animals.

The dorsal contour of the croup is convex and slopes toward the root of the tail, sometimes—commonly in the Lipizzaner and Belgian breeds—so steeply as to merit the description “goose rump.”

THE VERTEBRAL COLUMN

The vertebral column comprises 7 cervical, 18 thoracic, 6 lumbar, 5 sacral, and about 20 caudal vertebrae. Variations in number are not uncommon; the most frequent is the reduction of the lumbar vertebrae to five, especially in the Arab. The impression of shortness in the loins in other breeds is more often due to a marked caudal inclination of the last ribs.

The vertebral column inclines ventrally below the withers to reach its lowest point at the cervicothoracic junction, although the external elevation creates a contrary impression. It then changes direction abruptly, and as it ascends toward the poll, it shifts closer to the dorsal contour (see Figure 19–1).

The cervical vertebrae are individually long. Those behind the axis have rudimentary spinous processes, large divided transverse processes, and broad articular surfaces. The thoracic vertebrae are unremarkable apart from the great length of the spinous processes that form the basis of the withers. Independent centers of ossification develop for the summits of the first 12 or so spinous processes, and these may not fuse until comparatively late (10 or more years), if at all. The lumbar vertebrae have long horizontal transverse processes; synovial joints sometimes develop between those of the fourth and fifth bones and are constant between the fifth and sixth bones and between the sixth bone and the wings of the sacrum. In saddle horses exostoses sometimes develop on the summits of the thoracal spinous processes (mostly 14th-17th), bringing these into painful contact with their neighbors (“kissing spines”) and resulting in minor local deflections of the vertebral axis.

The intervertebral disks are relatively thin, collectively accounting for only 10% to 11% of the length of the vertebral column. Each consists of a peripheral anulus fibrosus and a central nucleus pulposus, but the boundary between these parts is less distinct than in many species. Age changes include dehydration and fragmentation of the outer fibrous part but rarely calcification of the center. The disks most severely affected tend to be those of the neck and that between the last lumbar vertebra and the sacrum, which are the regions where movement is greatest. The clinical importance of these changes is not clear.

The nuchal ligament, which divides the dorsal cervical muscles into right and left groups, is massively developed and supports much of the burden of the head without interfering with the ability to lower the neck when grazing (Figure 19–3). It consists of two clearly defined parts, each paired. The dorsal (funicular) part is a thick cord extending between the highest spines of the withers and the external occipital protuberance of the skull. It is flattened at its cranial attachment, becomes rounded shortly behind this, and flattens again as it nears the withers, where it forms a broad flange extending almost to the scapular cartilage. It is continued behind the withers as the narrower supraspinous ligament. The second (laminar) part forms a fenestrated sheet closely applied to its fellow. It fills the space between the funicular part and the cervical vertebrae and consists of bundles of elastic fibers that run cranioventrally from the funicular part and the spines of T2 and T3 to attach to C2–C7. Synovial bursae are interposed between the funicular part and certain bony saliences to minimize pressure. One, the nuchal bursa, is constantly present above the dorsal arch of the atlas; a second is sometimes found above the spine of the axis; and a third, the supraspinous bursa, is constantly present over the most prominent processes of the withers (Figure 19–3/2,2′,2″). Infections of the first and third, leading to conditions known as “poll evil” and “fistulous withers,” respectively, were formerly frequent and required extensive surgery for their eradication.

The complicated arrangement of the powerful epaxial muscles of the back and neck conforms, but only in a general way, to the account given in Chapter 2 (pp. 47–48). The many features of difference are fortunately not of clinical importance, and illustration of their arrangement in transverse sections of the neck and back will suffice for a description (see Figure 18–38). One specific feature of the associated deep fascia does, however, require notice. In the horse this thoracolumbar fascia possesses, opposite the scapula, an additional superficial lamina of importance. This, the dorsoscapular ligament (Figure 19–3/5,5′), has an origin, in common with the deeper layers, from the supraspinous ligament over the highest spines of the withers. In its ventral passage it is applied to the deep surface of the rhomboideus and gradually transforms from a purely fibrous to a largely elastic nature. It detaches a number of side branches that insert on the deep face of the scapula, alternating with divisions of the serratus ventralis muscle. The arrangement provides an elastic mechanism that helps absorb shock when the foot strikes the ground, limiting the dorsal shift of the scapula that would otherwise occur.

As always, the cervical part of the vertebral column is most mobile; the mouth may be brought around on full lateral flexion of the neck to reach the flank and on ventral flexion to reach the pasture. The latter movement is not always so easy for draft animals, which have relatively short necks; these animals may adopt a spreading posture of the forelimbs and may lean forward when grazing. Only small movements are permitted to the back and loins except at the very mobile lumbosacral joint.

THE VERTEBRAL CANAL

The relationships of the segments and cervical and lumbar swellings of the spinal cord to the vertebrae are shown in Figure 8–15. The first three sacral segments occur within the last lumbar vertebra, and the cord terminates within the cranial quarter of the sacrum of the adult (Figure 19–4).

The meninges remain separate to a more caudal level than in other species, and there is still a substantial subarachnoid space at the lumbosacral level. A communication exists in this species between the lumbar part of the space and a local widening (ventriculus terminalis) of the central canal of the spinal cord.

Both lumbosacral and caudal sites of injection are commonly employed to obtain epidural anesthesia. The procedure at the former level utilizes the divergence of the spinous process of the last lumbar and first sacral vertebrae for identification of the injection site (see Figure 19–4). Although the interarcuate space is quite large, its distance (8 to 10 cm) from the skin makes it relatively easy to miss. “Low” epidural anesthesia is performed between the first and second caudal vertebrae; the joint between these bones is very mobile, and the site for injection is readily discovered by “pumping” the tail up and down. The needle is inserted with a cranial inclination so that its point enters the canal within the first tail vertebra.

The vascularization of the spinal cord appears to be relevant to the etiology of a relatively frequent form of ataxia (“wobbles”) that occurs in foals and young horses. This may have its origin in congenital maldevelopment and subsequent exostoses of the cervical articular processes that narrow the vertebral canal at the intervertebral levels. These exert pressure on the cord, although it is said that the cord lesions are secondary to interference with the venous drainage. In this context it should be known that the spinal arteries and veins are arranged in two sets, connected by relatively ineffectual anastomoses. One set enters the cord by way of the ventral fissure and supplies (and drains) the central gray substance and a thin surrounding shell of white. The second clambers over the lateral aspect to detach branches at intervals; these enter to supply (and drain) the bulk of the white matter (Figure 19–5). It is the veins of the second set that are supposedly compressed, leading to venous congestion and subsequent degeneration of the nervous tissue. It is claimed that the condition may develop in the fetus.