system
Given the following post, respond as if you're commenting on Reddit:
human
Do blind people's pupils become dilated in the dark?
gpt
There is an afferent (going to) and an efferent (coming from) limb, in cranial nerves 3 and 4,respectively. So light comes in, sends a signal down the occulomotor nerve (CN3), then the signal gets sent down the trochlear nerve (CN4) to the iris. This stimulates the iris sphincter to contract and serves to constrict the pupil. 

The thing to know about this is that it's a carefully choreographed system, with each part being necessary for the other parts to respond appropriately. So if your retina is shot, your brain won't get the appropriate afferent signal and thus you won't get the constriction. If there is a reason for the blindness somewhere else in the body (brain, perhaps in the visual cortex), the necessary pathways may not be disrupted, resulting in maintenance of the dilation or constriction response of the pupil.