Half the Window
Asystole

We draw the window, left and right,
Two eyes sharing one patch of light.

Each eye sees both halves of the view,
Retinal images turn it through.
Nasal retina sees the outer field,
Temporal retina sees the inner.
Nasal fibres cross at the chiasm,
Temporal fibres stay their side.
Optic tract, then lateral geniculate,
Radiations carry light behind.

Right field, left brain,
Left field, right brain.
Two eyes share each half we see;
Trace the field carefully.
Nasal fibres cross the middle,
Temporal fibres stay in line.
Find the missing half of the window,
Follow it back, one step at a time.

A completely damaged optic nerve
Takes vision from that eye away.
A central chiasm lesion can take
Both outer fields from the day.
Behind the chiasm, a one-side lesion
Can lose the opposite half in both eyes.
Homonymous hemianopia:
Same field-side missing, side by side.

Right field, left brain,
Left field, right brain.
Two eyes share each half we see;
Trace the field carefully.
Nasal fibres cross the middle,
Temporal fibres stay in line.
Find the missing half of the window,
Follow it back, one step at a time.

Meyer's loop through the temporal lobe
Carries the upper opposite view.
Damage there: upper quarter missing,
In both eyes, the same quarter too.
Parietal fibres carry the lower;
Damage loses that quarter across.
Occipital cortex may spare the centre,
Even with a half-field lost.
We mark the field from the viewer's side,
And keep each eye clear on the page.

Right field, left brain,
Left field, right brain.
Two eyes share each half we see;
Trace the field carefully.
Nasal fibres cross the middle,
Temporal fibres stay in line.
Find the missing half of the window,
Follow it back, one step at a time.

The window holds the morning blue,
One world arriving through eyes of two.