Vasculature of the Equine Brain - Horse Anatomy

Circle of Willis

Blood is supplied to the brain from a ventral arterial supply in all species; from a circle of arteries called the Circle of Willis (also called the cerebral arterial circle or arterial circle of Willis) which lies ventrally to the hypothalamus where it forms a loose ring around the infundibular stalk. Blood is supplied to the brain by the internal carotid artery in horses. The Circle of Willis is made up of five main pairs of vessels:

• Rostral Cerebral Arteries: supply the medial aspect of the cerebral hemispheres.
• Middle Cerebral Arteries: supply the lateral and ventrolateral aspects of the cerebral hemispheres.
• Caudal Cerebral Arteries: supply the occipital lobes.
• Rostral Cerebellar Arteries: supply the rostral aspects of the cerebellum
• Caudal Cerebellar Arteries: supply the caudal and lateral aspects of the cerebellum.

The arrangement of the Circle of Willis means that if one part of the circle becomes blocked or narrowed (stenosed), or one of the arteries supplying the circle is stenosed, blood flow from the other blood vessels can continue to provide a continuous supply of blood to the brain.

The main blood supply to the circle is via the paired internal carotid arteries and the basilar artery. The basilar artery receives blood from the ventral spinal artery and the vertebral artery (the vertebral artery is a branch of the subclavian artery running through the vertebral foramina of C1 - C6). The maxillary artery does not contribute to the arterial circle in the horse, but it does supply the meninges. In horses, the vertebral artery can also supply the internal carotid artery via the occipital artery but this can be bypassed so that the vertebral artery can directly supply the internal carotid artery via a ramus to the internal carotid directly from the vertebral artery.

Rete Mirable

The brain is particularly susceptible to increased blood temperature and to protect the brain from any potential heat stress a number of species have developed protective mechanisms with the ability to selectively cool the brain. This protective system is often referred to as the Rete Mirable. The Rete Mirable is a complex network of arteries and veins lying very close to each other and depends on a countercurrent blood flow between the arterioles and venules (blood flowing in opposite directions). It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient.

Venous Sinuses

Venous sinuses drain the brain, meninges, and surrounding bone as well as participate in cerebrospinal fluid resorption; they are arranged into two systems. The dorsal system is within the dura mater of the cranium and drains the cerebral cortex, the cortex of the cerebellum, the deeper telencephalon, part of the diencephalon, and the tectum of the midbrain. The ventral (basilar) system lies on the floor of the cranial vault and drains the brainstem. The dorsal and ventral systems have minimal connection between them, but each communicates with the extracranial venous system. The dorsal system begins where several dorsal cerebral veins converge in the area of the crista galli of the cribriform plate.

The dorsal sagittal sinus arises from this convergence and runs caudally along the dorsal midline; surrounded by the falx cerebri as it lies against the skill bones. Along its course, it receives cerebral veins, meningeal veins, and diploic veins from the skull. The dorsal sagittal sinus of the horse, is incompletely divided by a septum and bifurcates rostral to the osseous tentorium. Just before reaching the osseous tentorium, the dorsal sagittal sinus receives a single sinus that drains the medial cortex, corpus callosum, basal ganglia, and part of the diencephalon. The transverse sinuses run ventrally from the osseous tentorium to the retroarticular foramen, where they exit to join the extracranial venous system. The transverse sinuses receive the dorsal petrosal sinuses, which mainly drain the rhinencephalon. They also receive veins from the caudal cerebrum, dorsal midbrain and the meninges. The transverse sinuses are connected via the communicating sinus without directly joining to the dorsal saggital sinus in the horse.

The ventral sinus system contains the cavernous sinuses, basilar sinus, and ventral petrosal sinus. The cavernous sinuses lie on either side of the pituitary gland on the floor of the cranial vault. They are joined across the midline by the cranial and caudal intercavernous sinuses to encircle the pituitary. This circle of sinuses around the pituitary has connections through the orbital fissure, the optic foramen, and the oval foramen to peripheral veins. The internal carotid artery lies within this sinus system in the horse. Caudally, the cavernous system communicates with the basilar sinus, which lies on the floor of the occipital bone, and the ventral petrosal sinus, which lies within the dura mater in the caudal part of the cranial vault. The ventral petrosal sinus exits the foramen lacerum or jugular foramen to become continuous with the jugular vein.