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 em.) 
long and an inch or more (2 to 3 em.) 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 turbinate part) is largely occupied by 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 
eayity. The fronto-maxillary opening is very variable in size. 
Squamous 
Squamous Parietal temporal 
part of occip- bone bone Frontal 
ital bone bone 
Lacrimal Mazx- 
bone illa 
Nasal bone 
Lateral 
part of 
occipital 
bone 
Premazilla 
Basilar part of 
occipital bone 
| 
Mandible 
Incisor teeth 
(first and second) 
Fie. 59.—Sxku.u or Foat Asour Two Monrus Op. 
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; 8, facial crest; 
9, infraorbital foramen; 10, mental foramen; 11, angle of mandible; 12, condyle of mandible; 13, coronoid process 
of 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 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 
(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 
