BLOOD VESSELS

 

The spinal cord receives its blood supply through spinal branches coming from the vertebral artery, the thoracic vertebral artery, the dorsal branches of the intercostal arteries and lumbar arteries. These arteries enter the vertebral canal through the intervertebral foramina.

 

The vertebral artery[1] vascularizes the cervical spinal cord; the thoracic vertebral artery[2] vascularizes the first thoracic segments; the dorsal branches of the intercostal arteries[3] vascularize the rest of the thoracic spinal segments; and the lumbar arteries[4] vascularize the lumbosacral spinal cord.

 

Within the vertebral canal, each spinal branch enters the subarachnoid space to form the dorsal and a ventral spinal artery. These arteries enter the subarcahomid space and run with the dorsal and ventral roots respectively to reach the spinal cord to form an irregular network that surrounds the spinal cord. Occasionally, a dorsolateral artery may be found on each side along the dorsolateral sulcus. Branches from this network (radial arteries) penetrate the spinal cord to vascularize the white matter and the outer regions of the dorsal horn. Ventrally, the network forms the ventral spinal artery that runs longitudinally in the ventral median fissure. It extends into the cranial cavity becoming the basilar artery. Along the spinal cord, it sends off segmental vertical arteries into the ventral median fissure (the vertical arteries) to vascularize most of the gray matter.

The venous drainage is provided by venous capillaries that drain to the network of veins that surrounds the spinal cord in the subarachnoid space. This venous network is connected to a dorsal and a ventral spinal veins that run with the dorsal and ventral roots of the spinal nerves.

 

The dorsal and ventral spinal veins that follow the spinal roots and drain to the internal vertebral venous plexus[5] formed by two great veins. Each vein that forms the internal vertebral venous plexus continue into the cranium as the basilar sinus (a venous sinus of the dura mater) inside the condyloid canal. It has to shift its position from ventral, in the floor of the vertebral canal, to lateral, between the outer and inner layer of the spinal dura mater at the level of the first cervical segments.

 

The internal vertebral venous plexus reaches the caudal vertebrae where it ends up joining the vertebral body or venules that are continuous with the tail. It consists of two parallel veins with no valves, located on the floor of the vertebral canal, inside the epidural space. The two veins diverge at the level of the intervertebral disc and converge over the vertebral bodies where anastomoses may be present dorsally and ventrally to the dorsal longitudinal ligament or located in the vertebral body (Page 714 of "Miller's anatomy of the dog" by Evans, H.E. 3rd ed.). In dogs, the diameter of the internal vertebral venous plexus is large at the cervical level, being the largest inside the atlas. The diameter is reduced at the cervicothoracic region and remains constant until the fourth or fifth lumbar vertebrae where they decrease in size.

The internal vertebral venous plexus receives the basivertebral veins[6] and the spinal veins. As the plexus has no valves, the venous blood flow direction in the internal vertebral venous plexus may vary depending on the pressure (Page 713 of "Miller's anatomy of the dog" by Evans, H.E. 3rd Ed.). It acts as a vascular bypass during transient increases in thoracolumbar pressure (Page 34 of "Imaging studies of the canine cervical vertebral venous plexus" by Gómez Jaramillo, M. A. Thesis dissertation 2005).

 

At the level of the interarcuate spaces, interspinous veins cross the ligamenta flava to enter the vertebral canal, and branch to form the interarcuate branches that join to the intervertebral veins at the thoracic level but they are absent caudal to T9. At the cervical region, they join the ventral internal vertebral plexus. 

 

TThe intervertebral veins are located at the intervertebral foramina and connected with the internal vertebral venous plexus. The first intervertebral veins are single but most are double. The spinal artery runs close to the spinal nerve in the rostral portion of the interventricular foramen. It sends off the spinal artery that enters the vertebral canal to form the spinal root arteries. The intervertebral veins are located caudally to the spinal nerve in the intervertebral foramen. The intervertebral foramina are closed by a membrane formed by two layers: internally, an outer layer of the dura mater (periosteal dura), and, externally, the deep sheet of the thoracolumbar fascia. This membrane is reinforced by crossed transforaminal ligaments and ligaments that attach to the spinal nerves, the spinal artery and veins. They serve to protect the nerve roots and vessels mechanically in stretching situations. Foraminal inflammation and fibrotic adhesion may be a potential cause of radicular pain by entrapment of the nerve roots.

 

The anastomoses between intervertebral veins and interspinous veins form the dorsal external vertebral venous plexus. In cervical and lumbar regions, ventral anastomoses of the intervertebral veins form the ventral external vertebral venous plexus.

 

The spinal root arteries and veins run with the spinal roots in the subarachnoid space being the ventral vessels bigger than the dorsal ones. The biggest ventral root arteries are the left and right C1 and C3, and left L4 and right L5. Not all the spinal segments receive a dorsal and a ventral root artery and vein. Some lack veins, some lack arteries, some lack the dorsal root vessels, some lack the ventral root vessels as it is shown in the picture.

[1] This arises from the subclavian artery.

[2] This arises from the costocervical trunk (branch of the subclavian artery).

[3] These arise from the thoracic aorta.

[4] These arise from the abdominal aorta.

[5] It is also called spinal sinuses or vertebral sinuses.

[6] The basivertebral veins are usually paired veins. They originate in the vertebral bodies and anastomose with the internal vertebral venous and the ventral external vertebral plexuses. They may be absent in the first segments of the thoracic region. The sacral and caudal vertebrae usually have no basivertebral veins (Page 714 of  “Miller’s anatomy of the dog” by  Evans H. E. 3ª Ed).