"Hollow cylinders" refers to objects that are shaped like a tube or pipe, but have an empty space inside rather than being solid throughout. Full definition
Its 19 hollow cylinders of resin sit directly on the floor. (mysanantonio.com)
Microtubules are hollow cylinders with walls made up of tubulin proteins — alpha (green) and beta (blue)-- plus EB proteins (orange) that can either stabilize or destabilize the structure of the tubulin proteins. (sciencedaily.com)
Illustration shows complex biostructure of dentin: the dental tubuli (yellow hollow cylinders, diameters appr. (sciencedaily.com)