Decussation

This chapter on “Decussation” examines the right–left crossing of neurological systems. Covered are the corticospinal tract, optic chiasm, and other subjects. The presence of crossed neurological systems is basic to clinical neurology. Crossing, however, appears to not be essential, and the amount of crossing can vary from individual to individual. We can track across vertebrate species the evolution of complete chiasmal crossing to the diminished crossing seen in the human visual system. This change in crossing of vision is very understandable as a correlate of the evolution of a lateral-eyed animal to a frontal-eyed human. The origin of crossing cannot be determined with certainty; we can only speculate about how many times crossing developed in pre-vertebrate history or what advantages, if any, crossing conferred. Clinicians, however, must be prepared to recognize patients with uncrossed anatomy—a challenge when we expect systems to be crossed as usual.