142 OUTLINES OE EQUINE ANATOMY. 



palate, as far as the soft palate, where it terminated in a 

 cul-de-sac. Some of the bones of the head present large 

 cavities lined by mucous membrane, which is (normally) 

 thin and transparent. These, the 



Sinuses of the head, are five in number on each side. 

 They present numerous bony trabeculse projecting inwards 

 forming partial septa, and are best developed in old 

 animals, being the result of absorption of bone. By con- 

 tainmg rarefied air they serve to lessen the weight of the 

 head, and by causing bulging of the bone they afford in- 

 creased surface for muscular attachment. The frontal 

 sinus in the bone of the same name is bounded by its 

 cranial, nasal, and external plates. Internally it is separated 

 from its fellow by a bony plate. To open this sinus we 

 draw a line from the lower margin of the orbital process 

 of one frontal bone to that of the other ; bisect this line and 

 in the angle so formed inferiorly trephine. The sphenoid 

 sinus (which only appears very late in life) and the ethmoid 

 sinus are cavities mainly contained in the bodies of the bones 

 of the same names respectively. The antrum or superior 

 maxillary sinus is formed by the palatine, superior maxil- 

 lary, malar, lachrymal, and superior turbinated bones. It 

 presents two bony cauals running through it — one, canalis 

 infra-orbitale, through which the superior dental artery and 

 nerve run ; the other is the ductus ad nasum, which termi- 

 nates by piercing the nasal plate of the superior maxilla 

 several inches up the nasal chamber ; being continued by a 

 membranous duct which runs underneath the mucous mem- 

 brane to its opening. The above sinuses open by a common 

 foramen into the nasal chamber, by a very small opening at 

 the posterior part of the middle meatus. The inferior max- 

 illary sinus is more anteriorly placed, and is sej)arated 

 from the superior maxillary sinus by a thin but imperforate 

 plate which extends to the opening into the nasal chamber, 

 thus dividing it into two. So this sinus does not com- 

 municate with the rest, all of which communicate freely 

 among themselves. To trephine this sinus, a line is drawn 

 parallel to the zygomatic ridge about an inch above it, and 

 the point where this meets, a perpendicular line connecting 

 the two from the termination of the ridge, indicates the 

 point of incision. 



The contents of the orbit may be examined by removal of 

 the superior and outer walls by sawing through the rest of 



