The =frontal sinus= is very large. It involves almost all of the frontal bone and a large part of the posterior wall of the cranium. It also extends for a variable distance into the horn processes when these are present. A complete median septum separates the right and left sinuses. The anterior limit is indicated by a transverse plane through the middle of the orbits. It extends externally to the crest which limits the temporal fossa above, and into the root of the supraorbital process. At the highest part of the cranial cavity and at the tuberosity on the posterior surface the two plates of the bone come together. The cavity is very irregular and is subdivided into numerous spaces by ridges and partial septa. This multilocular character is most marked in the anterior part, and here several small spaces appear to be cut off from the main cavity (Baum). The supraorbital canal passes through the sinus. Several small openings lead from the sinus to the ethmoidal meatuses, and thus indirectly to the upper division of the middle meatus nasi. The communications with the cavity of the superior turbinal and with the lacrimal part of the maxillary sinus which are seen in the macerated skull are closed in the fresh state by mucous membrane. [Illustration: FIG. 98.—SKULL OF OX, LATERAL VIEW WITHOUT MANDIBLE. The maxillary, lacrimal, and turbinal sinuses have been opened, and a portion of the orbit removed, _a_, Cavity of superior turbinal bone; _b_, lacrimal sinus; _c_, maxillary sinus; _d_, communication between maxillary and palatine sinuses; _e_, opening between maxillary and lacrimal sinuses; _f_, thin osseous bulla; _g_, lacrimal bulla; _h_, orbit; _1–6_, cheek teeth; _7_, nasal bone; _8_, premaxilla (nasal process); _9_, maxilla; _9′_, infraorbital foramen; _10_, frontal bone; _11_, lacrimal bone; _12_, malar bone; _13_, fissure between nasal bone and maxilla; _14_, temporal bone (squamous); _15_, external auditory meatus; _16_, styloid or paramastoid process; _17_, occipital condyle; _18_, palate bone (perpendicular part); _19_, pterygoid bone (hamulus); _20_, tympanic part of temporal; _20′_, muscular process of petrous temporal. (After Ellenberger, in Leisering’s Atlas.) ]