Frontal Air Sinuses

In common with most primates, humans and apes have pneumatized bones (that is, bones possessing air sinuses) in the facial skeleton and cranial vault. However, the presence of air sinuses in certain bones, and the size and position of those sinuses, is variable across the hominoids. Among the more variably expressed sinuses are the frontal sinuses, which lie deep to the brow ridge (or in the case of humans that lack a pronounced brow ridge, the supraorbital area) near the midline of the face. These sinuses grow out of small air cells in the anterior portion of the ethmoid bone, and communicate with the nasal cavity via small openings (ostia) into the nasal cavity (or indirectly via the ethmoidal air cells). The close developmental, anatomical and physiological relationship of the frontal sinuses, along with those of the ethmoid and sphenoid bones, is reflected in their classification as “paranasal” sinuses. Despite having small-to-nonexistent brow ridges, humans have relatively large frontal sinuses (although this feature is highly variable across populations). Chimpanzees tend to have smaller frontal sinuses, while those of the gorilla are smaller still and orangutans lack them altogether. The functional role of the frontal sinuses is unclear, and they may simply be architectural hollows in parts of the face that are largely free of masticatory stress (and therefore do not require the reinforment of solid bone).