The foregoing statements refer to the arrangement in the average adult animal. In the foal the cavity (with the exception of its turbinal part) is largely occupied by the developing teeth. In horses five to six years of age the maxillary part of the sinus is still filled up to a large degree by the embedded parts of the teeth. As the teeth are extruded to compensate the wear, more and more of the cavity becomes free, until in old age only the short roots project up in the floor, covered by a layer of bone. Other facts in this connection will be given in the description of the teeth. In exceptional cases the posterior part of the inferior turbinal is smaller than usual and leaves a considerable interval, through which the maxillary sinus communicates with the nasal cavity. [Illustration: FIG. 39.—SKULL OF HORSE, LATERAL VIEW WITHOUT MANDIBLE. THE SINUSES ARE OPENED UP. _a_, Posterior part, _b_ and _c_, anterior part of frontal sinus; _d_, roof of superior meatus; _e_, lateral mass of ethmoid bone; _f_, _f′_, naso-lacrimal duct, exposed in its posterior part; _g_, _h_, posterior and anterior compartments of maxillary sinus (also designated as superior and inferior maxillary sinuses); _i_, septum between _g_ and _h_; _k_, lower limit of upper thin and partly membranous portion of septum; _l_, infraorbital canal; _m_, turbinal part of maxillary sinus; _n_, bullous prominence of inferior turbinal; _o_, orbit; _p_, infraorbital foramen; _q_, continuation of infraorbital canal to premaxilla; _r_, limit of maxillary sinus. (After Ellenberger, in Leisering’s Atlas.) ] The =frontal sinus= (Sinus concho-frontalis) consists of frontal and turbinal parts. The frontal part is bounded chiefly by the two plates of the frontal bone, but its floor is formed in part by the lateral mass of the ethmoid. It extends forward to a plane through the anterior margins of the orbits, backward to one through the temporal condyles, and outward into the root of the supraorbital process. It is separated from the sinus of the opposite side by a complete =septum=. It is partially subdivided by a number of bony plates. The turbinal part is situated in the posterior part of the superior turbinal bone, roofed in by the nasal and lacrimal bones. It extends forward to a transverse plane about half-way between the anterior margin of the orbit and the end of the facial crest. Behind it is in free communication with the frontal part over the lateral mass of the ethmoid. It is separated from the nasal cavity by the thin turbinal plate. The frontal and maxillary sinuses communicate through the large opening described above.