Fig. 6.2 The back is relatively inflexible from withers to tail

The skeleton (Figs 6.3, 6.4)

The vertebral column is the skeleton of the neck, withers, back and tail. Besides support, the main function of the vertebral column is to provide protection for and house the spinal cord, which carries nerve impulses to and from the brain.

Fig. 6.3 Vertebral column

Fig. 6.4 The back from above Withers

Regions of the spine

The vertebral column can be divided into five regions

• The neck – 7 cervical vertebrae
  
• Upper back – 18 thoracic vertebrae
  
• Loins – 6 lumbar vertebrae
  
• Croup – 5 fused sacral vertebrae
  
• Tail – 15–20 coccygeal vertebrae.

Vertebrae

The vertebrae make up a long bony chain to protect the spinal cord. At each vertebra a pair of spinal nerves branches off from the spinal cord to penetrate every part of the horse’s body. Muscles are attached by their ligaments to the lateral and articular processes of the vertebrae so enabling the horse to move. The spinal cord ends in the middle of the sacrum where it sends out nerves to supply the horse’s tail.

Structure of the vertebrae

Each vertebra has the same basic shape:

• The vertebral main body or centrum
  
• An arch surmounted by the dorsal spine
  
• A pair of transverse processes of variable size and shape
  
• Two pairs of articular surfaces.

Intervertebral discs

The degree of movement of the spine depends on the thickness of the intervertebral discs that are firmly attached between the vertebrae. As the horse gets older the discs become calcified, thus joining the vertebrae together. There may even be further outgrowths of bone acting as bridges across neighbouring vertebrae.

Cervical vertebrae

The cervical vertebrae were described in Chapter 2.

Thoracic vertebrae (Fig. 6.5)

There are 18 thoracic vertebrae, each separated by cartilaginous intervertebral discs. The spinous processes are very large, giving the horse its pronounced withers and allowing extensive muscle and ligament attachment.

The withers: The withers are the highest point of the thoracic spine and are formed by the third to the tenth thoracic vertebrae. The withers are held firmly in place by ligaments between the vertebral spines and other muscles and ligaments attached to them, including part of the nuchal ligament.

Movement between the horse’s thoracic vertebrae is strictly limited.

Lumbar vertebrae (Fig. 6.6)

The lumbar vertebrae make up the loins region. There are normally six lumbar vertebrae but sometimes only five; in same breeds, particularly the Arab, an extra thoracic vertebra is often found.

The width of the transverse processes and the length of the dorsal spines characterise the lumbar vertebrae. The lumbar vertebrae of the horse carry three extra articular facets, making the horse different from most other mammals; these limit movement of the spine in the lumbar region. However, it should be remembered that:

Fig. 6.5 Thoracic vertebra – side view

Fig. 6.6 Lumbar vertebra – side view

• The loins are the most flexible and vulnerable part of the back.
  
• A well-designed and correctly fitting saddle, ensuring the rider’s weight is not taken on the loins or the vertebral column, will help to protect the horse’s back.

Fig. 6.7 Sacrum – view through the pelvis

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