THE VERTEBRAL COLUMN AND BACK
The vertebral formula is usually given as C7, T14-15, L6-7, S4, Cd20-23, but variation outside this range is common and almost always affects the thoracolumbar region where the total number of vertebrae varies between 19 and 23.
An increase in number is more common, possibly the result of selective breeding for this character: the loins are the most valuable part of the carcass, apart from the hams (Figure 33-1).Among other features, the vertebrae of the cervical region are distinguished by a high spine on C2 and a very high one on C7. Since the neck is almost as deep as the cranial part of the thorax, the body of the first thoracic vertebrae is located near the middle of the trunk at this level. The vertebrae behind the first rise gradually until those of the caudal thoracic and lumbar regions run close to, and almost parallel with, the dorsal contour of the back. The four units of the sacrum lack spinous processes; thus, there is an abrupt drop in the height of the vertebral column at the lumbosacral junction. The iliac crest, which flanks the spinous process of the last lumbar vertebra, is the highest skeletal feature in this area (see Figure 32-1/52).
The lumbosacral space is available but rarely used for the epidural administration of anesthetic (Figure 8-56, C). It measures about 2 cm craniocaudally and 3 cm transversely and is situated between 2 and 5 cm caudal to the line connecting the coxal tubers, which are palpable in less fat animals. If this guide cannot be used, an indication of the location of the lumbosacral space is provided by the transverse plane of the flank fold. The space is 5 cm or more below the skin, and the arrival of the needle point at the interarcuate ligament is made known by the greater resistance encountered there. In young hogs, the spinal cord extends into the sacrum and is at risk in this procedure; in older animals the ascent of the cord carries it to safety within the lumbar part of the canal.
The most caudal vertebrae are incorporated in the curly tail, which carries the median caudal vessels near its ventral surface.
Blood may be collected most easily at the tail head (Cd 4 or 5), but because the artery and accompanying veins run together, it cannot be predicted whether this blood will be of arterial, venous, or mixed origin. The tail is often removed when a piglet is a few days old to prevent the common vice of tail-biting, which sometimes results in ascending infection. Trichinosis (occurring in some countries) may also be transmitted in this way.The contour of the back depends on breed and condition. In fat, old animals it may be flat, but in most modern hogs it is uniformly arched and, in those of top quality, also broad. A broad back and wide stance promise good muscling of the trunk and thick hams. The muscles of the back conform to the common pattern, and the longissimus (“loin eye”) and, most especially, psoas muscles (filet mignon) constitute particularly valuable parts of the carcass. Since subcutaneous fat has limited value, too thick a layer is undesirable; this indication of carcass quality may be measured by ultrasound. That deposited over the loins is especially well-formed and thick, and because it has to be trimmed, it represents a substantial loss to the producer. Some of it is rendered into lard, and some is cured to become the “pork” in the popular canned food “pork and beans.” Selective breeding has markedly reduced to 3 cm or less the thickness of back fat; consequently, caution is needed when intramuscular injections are performed.