Chiasm (anatomy)

In anatomy a chiasm is the spot where two structures cross, forming an X-shape (from Greek letter χ, Chi).

The chiasm is distinguished from a decussation, which is a crossing of nerve fibres inside the central nervous system.

Since the midline crossing occurs inside the brain, it is not strictly a chiasm but rather a decussation.

The optic tract of cephalopods, such as the squid Loligo,[3] chiasmates without midline crossing.

This chiasm is distributed along the optic tract and effectively compensates the inversion of the image on the retina.

[6] Another example is the optic radiation which rotates the retinal map on the visual cortex by 180° (see Figure 3).

A number of theories have been proposed to explain the existence of the optic chiasm in vertebrates.

The lens eye inverts the visual image that is projected on the retina due to the camera obscura effect.

In jawless vertebrates (hagfish and lamprey), the optic tracts do cross in the midline, but only after entering the ventral side of the central nervous system.

Figure 2. Example of bifurcating axons in the optic chiasm (Type II) of a rabbit. a,b,c: bifurcating optic fibres. c: fibre bifurcating in the two opposite optic tracts. d. Commissure of Gudden. e. Fibres that continue in a different depth. (from: Cajal, [ 5 ] Fig. 6)
Figure 3. The decussation of the optic radiation in the cortex is an example of a type IV crossing