1. The visual system

1.1 The retina

As we've said, this is very thin neuronal tissue (0.1 to 0.5 mm thick) whose destruction by disease or trauma will be substantially definitive, since neurons are cells that do not reproduce during life.

Schematically, the retina comprises 3 main cell layers (figure 2 ).

• The deepest receptor layer (the human retina being "inverted", since the receptors are furthest away from the penetration of light rays) comprises around 100 million receptors, themselves of two types long distinguished by their shape: rods (around 95%) and cones (5%). The cones alone are present in the central region of the retina, forming a tightly...