Uncombable Hair Syndrome

Pili trianguli et canaliculi is a rare genetic disease also known as "uncombable hair syndrome" and "spun glass hair." From an abstract in the medical journal Ultrastructural Pathology (photo from The World's Fair blog):
 3172 2612405338 0F265Fc152 Both inherited (autosomal dominant and recessive with variable levels of penetrance) and sporadic forms of uncombable hair syndrome have been described, both being characterized by scalp hair that is impossible to comb due to the haphazard arrangement of the hair bundles. A characteristic morphologic feature of hair in this syndrome is a triangular to reniform to heart shape on cross-sections, and a groove, canal or flattening along the entire length of the hair in at least 50% of hairs examined by scanning electron microscopy. Most individuals are affected early in childhood and the hair takes on a spun-glass appearance with the hair becoming dry, curly, glossy, lighter in color, and progressively uncombable. Only the scalp hair is affected.
Uncombable Hair Sydrome on Wikipedia, Uncombable Hair Syndrome on The World's Fair blog