VSR 421: Regional Veterinary Surgery (2+1)

 AMAUROSIS

• Amaurosis is blindness without any apparent lesion in the eye. It may be temporary or permanent.
• Possible causes are toxaemia, lesions in the brain, etc. (Note: A temporary form of amaurosis is sometimes seen in cattle due to deficiency of vitamin A which can be corrected by administration of vitamin A.)

Refraction of the eye

• Parallel rays
  • The amount of divergence of light rays falling on a given area is inversely proportionate to the distance from the source of light. When the distance is 20 feet or more, the divergence is so slight that the rays can be considered as parallel.
• Emmetropia (Normal sight)
  • When the refraction of the eye is normal, parallel rays coming into the eye in a condition of rest, are focused exactly on the retina. This condition is known as emmetropia.
• Ametropia
  • Ametropia is a term used to denote a condition of abnormal refraction of the eye due to hypermetropia, myopia, or astigmatism, in which parallel rays are focused either in front or behind the retina.
• Hypermetropia (Hyperopia; Long sight; Far sight)
  • Hypermetropia is a condition of abnormal refraction of the eye in which parallel rays come to a focus behind the retina. This type of ametropia is caused if the axis of the eyeball is too short or if the refractive power of the eye is too weak.
• Myopia (Short sight; Near sight)
  • Myopia is a condition of abnormal refraction of the eye in which parallel rays get focused in front of the retina. This may happen either due to the axis of the eyeball being too long or due to the refractive power of the eye being too strong. (In this condition the eye is able to see clearly only objects very close to it.)
• Astigmatism
  • When the refraction through several meridians of the eye is different, the condition is called astigmatism.
  • Agtigmatism may be caused by irregularities in the cornea or the lens. Astigmatism causes blurred vision. (Note: A certain degree of astigmatism is normally present in the horse.)