"Very small ones," Napier said reassuringly. "They are introduced 
through the blood, of course—the hæmocules circulate through the 
bloodstream until they find themselves passing through capillaries in the 
brain, at which point they cut through the blood/brain barrier and fasten 
themselves to a nearby axon. They can monitor activity in the axon or trigger 
it. These 'sites all talk to each other with visible light."