Brain vessels enter the central nervous system through holes in the meningeal layers. The cells in the blood vessel walls are joined tightly to one another, forming the blood–brain barrier, which blocks the passage of many toxins and pathogens (though at the same time blocking antibodies and some drugs, thereby presenting special challenges in treatment of diseases of the brain). Neuroanatomists usually divide the vertebrate brain into six main regions: the telencephalon (cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla oblongata. Each of these areas has a complex internal structure. Some parts, such as the cerebral cortex
Where the Boys Are to head the cast." Natalie Wood, who had just made "All the Fine Young Cannibals" for MGM, was mentioned as a possible star at one stage. MGM eventually persuaded the book's author to change the title from "Unholy Spring" to "Where the Boys Are". Henry Levin was signed to direct. The first two stars confirmed for the movie were George Hamilton and Yvette Mimieux. Paula Prentiss was cast despite never having made a movie before. Connie Francis also made her movie debut. The novel contained a section where the students help raise money to ship arms to Fidel Castro for