Fissure
A narrow opening between hills or mountains is referred to as a "pass" or a "mountain pass." These pathways provide a route through mountainous terrain, allowing for travel between two areas that may otherwise be difficult to access. Passes can vary in width and difficulty depending on the specific terrain features.
The shape of the opening. A Beaker has a wide opening, the curved sides of the beaker do not converge. A flask has a narrow opening, the curved sides form a narrow neck, that can be bunged.
A Gorge
A Gorge
Fissure
A narrow opening can be considered a slit. Narrow openings can be very long or short. There are many different types of narrow openings.
Ganae
Slot
The pharynx is the narrow opening at the back of the mouth. It is part of both the digestive system and the respiratory system.
Narrow
spinal cord
Blood vessels in the brain of a cocaine user will narrow so that means the vessels contract.