Becca then walked to Koenig's side and took her bundle of keys out of her pocket. She jangled them behind Koenig, and saw that he flicked his ears backward to see where the noise came from.
The 8th cranial nerve governs both hearing and balance, she explained to Suzie. The normal horse should have exquisitely acute hearing – and should react readily to any unexpected sound. The normal horse should also exhibit excellent balance (the vestibular portion of CN VIII). If the horse's head is cocked to one side, or if the eyes are moving rapidly back and forth (called nystagmus), then we may suspect vestibular disease – a problem with the horse's vestibular, or balancing system.
Suzie watched as Becca then offered Koenig a piece of carrot that she had in her pocket. Koenig eagerly chewed and swallowed the carrot, and looked for more.
That simple act of swallowing told me a lot, said Becca. The 9th cranial nerve – CN IX – helps to control the ability to swallow. The horse with a lesion in CN IX may not be able to swallow properly, and you may see food coming back out of the mouth despite the horse's ability to chew and obvious good appetite. The 10th cranial nerve, known as the vagus nerve, also helps to control swallowing, so I tested part of that nerve as well. CNX is also very important to the function of the larynx, or voicebox, so if Koenig had problems with CNX, you might notice that he had a funny-sounding whinny, or had started to be a 'roarer' – that is, had exercise intolerance and made a roaring noise when working hard because of laryngeal paralysis.
The 11th cranial nerve is harder to assess', Becca continued. He doesn't appear to have any asymmetry or atrophy of the trapezius muscle, so I'm assuming that CNXII is intact.
Becca then carefully reached inside Koenig's mouth and gently grasped his tongue. She really had to hold onto it to inspect it, because Koenig was pulling back hard.
CN XII controls the tongue, she explained. The tongue should have good musculature on both sides, and should stay inside the mouth – with the exception of a few horses who have a habit of lolling their tongues, or horses who have had an injury to the tongue. Horses have exceptionally strong tongues, and it should be difficult to grasp the tongue and gently pull it out of the mouth.
Dr. Smith was very pleased with Becca. She had done an excellent job of assessing the cranial nerves, and correctly found no abnormalities. They then decided to bring Koenig outside to finish the examination.
We've assessed a large portion of Koenig's brain function, said Dr. Smith. We still need to assess the ability of the cerebellum to control the action of Koenig's legs and indeed his entire body. The portion of the brain termed the cerebellum controls the ability of the horse to rate and control his actions. It lets the horse know how high, for instance, the foot should rise with each step, and how far forward and backward each limb should go. Horses with a lesion in the cerebellum will look a bit like a drunk three hours into happy hour – the horse will not be able to control how far each limb can go, and will stagger or over-reach dramatically. The horse will also show intention tremors – when the horse goes to perform an action intentionally, the cortical portion of the brain knows exactly what it wants to do – perhaps eat grain out of a bucket. However, the cerebellum doesn't tell the head when to limit where it is going, so it overshoots, then undershoots the location of the bucket – this action recurs very rapidly, and appears as a tremor. We can assess this best when Koenig is moving freely outside.
We also need to assess whether there is a problem at the level of Koenig's spinal cord. If we think of the central nervous system, or CNS, as a circuit, then think of the spinal cord as being downstream of the brain. The spinal cord has to transmit messages to the muscles in order for the horse to have a normal posture and gait. If there is a problem at the level of the spinal cord, then the horse may know perfectly well what he wants to do at the level of the cerebrum and cerebellum, but his muscles, and therefore his limbs, simply won't be able to carry out the brain's instructions.