Final answer:
The sympathetic nervous system increases cardiac contractibility and dilates the eyes.
Step-by-step explanation:
The part of the nervous system that increases cardiac contractility and dilates the eyes is the sympathetic nervous system. The sympathetic division of the autonomic nervous system is responsible for the fight-or-flight response, which includes increased heart rate and dilation of the pupils.
When light levels are low, the sympathetic system sends a signal to the iris, where it releases norepinephrine onto the radial fibers of the iris, causing pupil dilation. This allows more light to enter the eyes.