THE PARANASAL SINUSES 73 



three cheek teeth project up into the cavity to an extent which varies with age; 

 they are covered by a thin plate of bone. The cavity is divided into anterior and 

 posterior parts by an obhque septum. The outer margin of the septum is commonly 

 about one and a half to two inches (ca. 3.5 to 5 cm.) from the anterior end of the 

 facial crest; from here it is directed inward, backward, and upward. The upper 

 part of the septum (formed by the posterior end of the inferior turbinal bone) is 

 very delicate and usually cribriform. 



The position of the soptuin is quite variable. It is not rare to find it further forward, and 

 in some cases it is further l)ack than is stated above. In the recent state, i. < ., when covered l)y 

 the mucous membrane on both surfaces, it is nearly always complete, but in very exceptional 

 cases there is an opening of variable size in the upper part. 



The anterior compartment, often called the inferior maxillary sinus, is partially 

 divided by tlic iufruorl)ital canal into an external maxillary ])art and an internal 

 smaller turbinal part. The latter communicates with the middle meatus by a very 

 narrow slit situated at its highest part. The posterior compartment, often called 

 the superior maxillary sinus, is also crossed l)y the infraorbital canal, internal to 

 which it opens freely into the spheno-palatine sinus. It connnunicates dorsally 



Fig. .39. — Skull of Horsf.. L.\tfr.\i, View withoi-t MANnini.i:. Thk Sintsf.s ,\re Opened vp. 



a, Posterior part, b ami c, anterior part of frontal sinus; d, roof of superior meatus; e, lateral mass of eth- 

 moid bone; /, /', 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; /, 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 EUenberger, in Leisering's Atlas.) 



with the frontal sinus through the large oval fronto-maxillary opening, situated at 

 the level of the osseous lacrimal canal and the corresponding part of the inner wall 

 of the orbit; the orifice is commonly about one and a half to two inches (ca. 4 to 

 5 cm.) long and an inch or more (2 to 3 cm.) wide. Just in front of this, and covered 

 by a thin plate, is the narrow naso-maxillary fissure (Aditus naso-maxillaris) , by 

 which the sinus opens into the posterior part of the middle meatus. 



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 w^hich the maxillary sinus communicates with 

 the nasal cavity. 



The frontal sinus (Sinus concho-frontahs) 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 



