2. Intervertebral articulation:  Vertebrae form 2 sets of joints; One cartilagenous involving direct connection of the vertebral bodies, the other is synovial existing between articular processes and facets carried upon the vertebral arches, in addition to long ligaments extends over many vertebrae. Intervertebral discs:  They are thick but flexible pads that connect between vertebral bodies to allow slight movement of the vertebrae (provide flexibility and support) and acts as a ligament to hold the vertebrae together.

3.  They represent about 10% of the vertebral column length in ungulates, 16% in dogs & 25% in man.  There is no IVDs for the atlas, the axis, sacral vertebrae.  The IVD is thinner in the back than in the loins or neck, the disc between the last lumbar body and the sacrum is especially thick. Fig.1: Intervertebral discs in between the adjacent vertebral bodies.

4. The structure:  The IVD is flat, round structure that consists of two anatomically distinct regions: 1. An outer layer of fibrocartilaginous material (the anulus fibrosus) which consists of several layers of fibrocartilage & surrounds the inner nucleus pulposus and distributes pressure evenly across the disc. 2. An ovoid central region of gelatinous material (the nucleus pulposus).

5. Fig.2: Cervical vertebra with IVD. (Disc annulus labeled at bottom right, and is visible at center in blue).

6. Fig.3: thoracic vertebrae with intervertebral discs.

7. The anulus fibrosus:  It functions to produce stability between vertebrae while allowing some flexibility of the spine. It is important in shock absorption and keep the nucleus pulposus intact and prevents it from bulging outwards.  It is composed of annular fibrocartilage bands. These annular bands are arranged in a specific pattern to resist forces placed on the vertebral spines. The annular bands are subdivided into inner fibers, which are connected to the cartilaginous endplate that cap the bones, and outer Sharpy fibers, which are attached to the VB.  It is approximately two times thicker ventrally than dorsally, and its laminar structure is most prominent ventrally and least prominent dorsally.

8. The nucleus pulposus:  It contains loose fibers suspended in a mucoprotein gel, acts as a shock absorber, absorbing the impact of the body's daily activities and keeping the two vertebrae separated and transmits stress and weight from vertebra to vertebra.  The nucleus pulposus consists of water, collagen and proteoglycans, it develops from the embryonic primitive notochord. It has a very high water content, which keeps it moist.  When nucleus pulposus is forced out of disc, it may put pressure on the nerve located near the disc causing sciatica.

9. IVDs function:  These disks provide flexibility and support (cushion between the vertebrae) to spread the pressure placed on the spine and to absorb the shocks and movements of normal activity.  The discs expand while at rest allowing them to soak up nutrient rich fluid. When the process is inhibited through repetitive movement, injury or poor posture, the discs become thinner and more prone to injury. This may be a cause of the gradual degeneration of the structure and function of the disc over time.

10. Innervation:  IVDs innervated directly by sensory branches of the dorsal rami and recurrent meningeal nerves. Blood supply:  No blood vessels enter the IVDs, but they receive their blood supply through movement as they soak up nutrients.

11. IVD disease: IVD disease is common in humans and dogs but is rarely reported in horses. Fragmentation of the anulus fibrosus allow the nucleus to escape in the direction of the vertebral canal, where directly or in directly it may press on spinal cord and nerves (disc degeneration). This degeneration may affect any disc but the effects are likely to be most sever at those of the neck & lumbosacral junction.

12. Fig.4: Ruptured disc (see protrusion of the nucleus)

13. Fig.5: A lateral myelogram of a dachshund with a herniated intervertebral disc. Note the loss of contrast column indicating spinal cord compression caused by the ruptured intervertebral disc.

14. Synovial joints: In addition to the IVD, special joints between each of the vertebral bodies, called facet joints, allow the individual vertebra to move and rotate with respect to each other. The joints between the facets on the vertebral arches are conventional synovial joints. The joints between the articular processes are called (Juncturae zygapophyseales) that allow movements parallel to the articular surfaces. fig.6: Lumbar intervertebral joint.

15. The nature and degree of mobility vary with the region and to some extent with the species. The movement is most free in the neck where the articular surfaces are largest & capsules most loose. In the cervical and cranial thoracic regions rotation is possible in addition to the flexion and extension. The articular surfaces change their position so, rotation is possible in the cranial thoracic region. In the caudal thoracic and lumbar regions the movement decreases towards the caudal end.

16. Only dorsal and ventral flexion is possible between the last thoracic and lumber vertebrae. It is possible for the vertebral column to move laterally (skoliosis). Sacral vertebrae are fused due to ossification of the IVD, fusion of the vertebral arches and the roots of the spinous processes together. The connection between the sacrum and last lumbar vertebra is formed by ligaments, by the discs between the 2 vertebral bodies & by the Juncturae zygapophyseales lumbosacrales between the articular surfaces.

17. Horse also has joints between the wings of the sacrum and transverse process of the last lumbar vertebra (lumbosacral intertransverse articulation), and also between transverse processes of the last 2 lumbar vertebrae (lumbar intertransverse articulation). The caudal vertebrae are highly mobile as their terminal surfaces are convex with thick IVD.

18. costovertebral articulation:  Costovertebral articulation consists of 2 separate joints; one between the head of the ribs and the vertebral bodies, the other between the tubercle of the ribs and vertebral transverse processes. 1.Articulatio capitis costae: In this joint; the 2 articular surfaces of the rib head articulate with the facets of the adjacent vertebrae. 2.Articulatio costotransversaria: In this joint; the articular surface of the rib tubercle articulates with the facets of transverse process of the thoracic vertebra.

19. The articulation of the costal head are spheroid joints, while the articulation of the tubercles are amphiarthroses. The movement of these joints are performed simultaneously to widen or narrow the thoracic cavity. The nearer the joints are together, the greater the degree of movement, as in caudal ribs, so they are often called the respiratory ribs. Most thoracic intervertebral discs are crossed dorsally by intercapital lig. which passes through intervertebral foramen and unit the heads of the left and right ribs, so minimizes the disc rupture at these levels.

20. Fig.7: thoracic vertebrae; left lateral view.

21. Fig.8: 5th thoracic vertebra (Lateral view)

22. Fig.9: cross section of the vertebral column, show the costovertebral articulation.

23. Fig.11:Left 1st rib; median view.

24. Fig.12: A left lateral view showing a rib articulating with 2 vertebrae.

25. Ligaments of the vertebral column:  The vertebral column is joined together by ligaments that allow the vertebrae to bend and twist carrying the weight of the animal body and provide strength & flexibility for the vertebral column. 1.Dorsal longitudinal lig: It runs along the floor of the vertebral canal from the axis to the sacrum, but in carnivores it reaches the caudal vertebrae. It narrows over the middle of each vertebral body and widens when it crosses each intervertebral disc, where it helps support this structure.

26. 2.Ventral longitudinal lig: It runs along the ventral surfaces of the vertebral bodies from the midthoracic region to the sacrum. It also widens below and fuse the intervertebral discs. 3.Interspinous lig: It runs between the spinous processes help prevent the spine from bending forward excessively. fig.13: Paramedian section of lumbar vertebrae.

27. 4.Nuchal lig:  It consists of funicular part and laminar part; both of which are paired. A.The funicular part:  It arises from the external occipital protuberance on either side.  It passes over the 1 st 2 cervical vertebrae without attachment.  It units with the laminar part above the 3 rd cervical vertebra.  It continues as supraspinous lig. as far as the 12 th thoracic vertebra.

28. B. The laminar part:  It has serrated origin from the spinous process of the axis and the following cervical vertebrae.  It inserts into the 1 st thoracic vertebra and into the interspinous lig. Dog has only funicular part. Pig & cat don‘t have nuchal lig. but they have supraspinous lig.

29. Fig.14: Nuchal ligament and its parts.

30. 5.Other ligaments: A.The capsular ligaments: It binds the facet joints together. B.The intertransverse ligaments: It runs between adjacent transverse processes.