THE PARANASAL SINUSES 



85 



superior maxillary sinus, is also crossed by the infraorbital canal, over which it 

 opens freely into the sphenopalatine sinus. It communicates dorsally 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 medial 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 ea\'ity (with the exception of its turbinate part) is largely occupied bj' the developing 

 teeth. In horses five to six years of age the maxillary part of the sinus is 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 ventral turbinate is smaller than usual and 

 leaves a considerable interval, through which the maxillary sinus communicates with the nasal 

 cavity. The fronto-maxfllarj' opening is very variable in size. 



Squamous 

 Squmnous Parietal temporal 

 part of occip- bone bone Frontal 



ital bone i I hone 



Lateral 



part of 



occipital 



bone 



Max- 

 illa 



Nasal hone 



Prvmaxilla 



Basilar part of 

 occipital hone 



Incisor teeth 

 {first and second) 



Fig. 59. — Skull of Fo.il About Two Months Old. 

 This figure illustrates differential features of skull of young foal as compared with that of adult animal shown in 

 Fig. 28. 1, Occipital condyle; 2, paramastoid process; 3, mastoid process; 4, external acoustic meatus; 5, zygomatic 

 process of temporal bone; 6, supraorbital process of frontal bone; 7, zygomatic process of malar bone; S, facial crest; 

 9, infraorbital foramen; 10, mental foramen; 11, angle of mandible; 12, condyle of mandible; 13, coronoid process 

 of mandible. 



Mandible 



The frontal or fronto-turbinate sinus (Sinus frontalis) consists of frontal and 

 turbinate parts. The frontal part is bounded chiefly by the two plates pf 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 

 '(Septum sinuum frontalium). It is partially subdivided by a number of bony 

 plates.' The turbinate part is situated in the posterior part of the dorsal turbinate 

 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. It is in free communication behind with the frontal part over the 

 lateral mass of the ethmoid. It is separated from the nasal cavity by the thin 



