Overview
The equine brain (encephalon) fills a cranial cavity that is small compared with the head, which is mostly face: the long nasal region and the continuously erupting teeth take up most of the skull. The cranial cavity lies caudal to the orbits, under the nuchal crest and the frontal and parietal bones, and is continuous with the vertebral canal through the foramen magnum.
Parts
- Cerebrum: two hemispheres with many gyri, separated by the longitudinal fissure. The cortex integrates vision, behaviour and voluntary movement.
- Cerebellum: caudal to the cerebrum, over the fourth ventricle; it coordinates movement and balance.
- Brainstem: midbrain, pons and medulla oblongata, the origin of most cranial nerves.
- Blood supply: unlike cattle, the horse has no rostral epidural rete mirabile. The internal carotid artery reaches the cerebral arterial circle directly after running along the dorsal wall of the guttural pouch (the diverticulum of the auditory tube), together with cranial nerves IX to XII.
Clinical relevance
- Head trauma: a horse that rears and falls over backwards hits its poll; the pull of the ventral head muscles can fracture the basisphenoid and basioccipital bones, with haemorrhage into the brainstem, vestibular signs and blood from the nostrils or ears.
- Viral encephalitides: Eastern, Western and Venezuelan equine encephalomyelitis and West Nile virus, transmitted by mosquitoes, cause fever, behavioural change, ataxia and recumbency. Vaccines are available.
- Hepatic encephalopathy: ammonia and other toxins that the diseased liver fails to clear cause depression, head pressing, blindness and yawning.
- Leukoencephalomalacia: liquefactive necrosis of the cerebral white matter caused by fumonisins in mouldy maize (Fusarium verticillioides); it progresses quickly and is usually fatal.
- Rabies: must be on the differential list for any horse with rapidly progressing neurological signs; necropsy and sample handling require precautions.
- Euthanasia by captive bolt or free bullet: the point of aim is where the lines joining each eye to the base of the opposite ear cross, slightly above it, because the brain lies further caudal than the shape of the head suggests.