The outer plate of bone has been removed to show the sinuses. _a_,
Frontal sinus; _a′_, cranial plate of frontal bone; _a″_, anterior
part of frontal sinus, which is separated from remainder by a septum
(_b_); _c_, _c′_, communications between frontal sinus and nasal
cavity; _d_, supraorbital foramen; _e_, supraorbital canal; _f_,
cavity of superior turbinal bone, and _g_, its opening into the
nasal cavity; _h_, lacrimal sinus, _i_, its communication with the
maxillary sinus; _k_, maxillary sinus; _l_, orbit; _1_, frontal
bone; _1′_, processus cornu; _2_, nasal bone; _3_, premaxilla (nasal
process); _4_, maxilla; _5_, lacrimal bone; _6_, malar bone; _7_,
dotted line indicating course of naso-lacrimal duct. (After
Ellenberger, in Leisering’s Atlas.)
]
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: