THE VENTRAL PART OF THE NECK
The ventral part of the neck contains the visceral space occupied by the esophagus, trachea, and other structures passing between the head and the thorax. This space is bounded dorsally by the muscles below the vertebrae and laterally and ventrally by flatter muscles united by stout fasciae.
The foremost lateroventral muscles are the brachiocephalicus and sternocephali- cus, which bound the groove occupied by the (external) jugular vein (Figure 18-38/72). The caudal part of this groove is covered by the cutaneous muscle of the neck, which radiates from a manubrial origin; the muscle thins as it passes from its origin, which increases the prominence of the cranial part of the vein, the obvious target when the vein is raised for puncture (Figure 18-39/9,77). The brachiocephalicus is described on page 587.The right and left Sternocephalicus muscles arise from the manubrium side-by-side but diverge toward their mandibular insertions (Figure 18-39∕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 18-38/5).
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.”