Molera
A molera is a "hole" in a Chihuahua's head; it is the same as a fontanelle in human babies. Historically, the Chihuahua as developed in Mexico and the United States has displayed a "hole in the head". In times past, this has been accepted as a mark of purity for this dog breed, and it is still mentioned in most Chihuahua breed standards the world over. The bones of the head in all foetuses are not firmly knitted together, but in most mammals, the different bones of the skull join with cartilaginous sutures as the animal matures. The Chihuahuas' moleras vary in size and shape, occurring on the top of the head where the parietal and frontal bones come together.
The simple fact that a Chihuahua has a domed head with an open molera does not predispose it to hydrocephalus as found by a study done by Greene and Braund in 1989, which concluded that "Many clinically normal toy breeds and brachycephalic (short faced) breeds also may have open fontanelles without associated hydrocephalus." Also, "There did not appear to be any relationship between the presence or size of the fontanelle and the concomitant presence of hydrocephalus." When the dog seems normal but displays enlarged ventricles of the brain (under ultrasound), its hydrocephalus is termed "occult" (having no clinical signs).