Disclaimer: This blog is for educational purposes and please be warned that this blog contains pictures from real animal post mortems and graphic descriptions of disease.
Did you know that eyes just are outgrowths of the brain? They develop from cups of cells sitting on the forebrain. Most of the eye acts like a camera by focusing light onto the retina – the light sensitive layer of brain cells at the back of the eye. Signals from the retina travel up the optic nerve, which leaves the back of the eye and goes to the brain. If there is a problem with vision, we can work out where the damage is, by knowing the pathway the signals take. What’s interesting is that the optic nerves meet and ‘cross over’, to the opposite side of the brain.
The percentage of nerve fibres within each optic nerve that cross over depends on the species. In most fish, amphibians, reptiles and birds, 100% of fibres cross over, meaning that all vision from the left eye is processed by the right side of the brain and vice-versa. In other animals, there is a more even split of crossed and uncrossed nerve fibres so that each side of the brain receives signals from both eyes. This allows images from both eyes to be combined, giving us creatures with forward-facing eyes stereoscopic 3D vision with better depth perception.
There is even an animal that starts out with the nerves completely crossed, but later on ends up with split crossed and uncrossed nerves…