. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. edinger: frontal sinus evolution in the equidae 413 The right and left frontal sinuses are separated only by a double sheet of bone, the septum sinuum frontalium, whose thickness varies around 1 mm. The pair constitutes the largest pneumatic area in the horse skull. Among the numerous detailed investigations of these cavities, those based on the most material are those of Goubeaux 1852, Baum 1894 (66 skulls of 5-to-15 year-olds), and Bresson 1919 (40 fresh heads and 43 skulls, from 4 years to very old); a more recent
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. edinger: frontal sinus evolution in the equidae 413 The right and left frontal sinuses are separated only by a double sheet of bone, the septum sinuum frontalium, whose thickness varies around 1 mm. The pair constitutes the largest pneumatic area in the horse skull. Among the numerous detailed investigations of these cavities, those based on the most material are those of Goubeaux 1852, Baum 1894 (66 skulls of 5-to-15 year-olds), and Bresson 1919 (40 fresh heads and 43 skulls, from 4 years to very old); a more recent one is Anthony 1929, and shorter descriptions are found in textbooks such as Sisson 1940. These studies have established the following condi- tions in the adult common horse. Each frontal sinus lies primarily between the tabulae externa and interna of the frontal bone. Laterally it extends into the supraorbital (zygomatic) process of the frontal. Posteriorly it may reach almost to the fronto-parietal suture (14% of Bresson's cases). Anteriorly it has a dorsal extension into the nasal bones. Laterally, too, it continues. Fig. 1. Equus caballus. Topography of left frontal and maxillary sinuses. After Bressou (slightly changed). }4 nat. size. beyond the frontal bone — along the ethmoidal labyrinth, roofed in by the lacrimal, nasal, and maxillary bones. Its osseous floor is inter- rupted by a wide opening, up to 45 mm. long and 35 mm. broad (pi. 1 = fig. 8, — a communication with the maxillary sinus. Anterior to this there is a tapering anterad section of the frontal sinus. This extends into the posterior part of the dorsal (naso-) turbinate bone. It is anteriorly closed off, by a transverse wall, from the anterior part of the dorsal turbinate (which part alone functions, with scrolls, as a nasal turbinate). This anterior extension of the sinus is often called pars turbinalis sinus frontalis. Sinus nomenclature varies con- siderably, depending on the importance an author a
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