VETERINARY DENTAL SURGERT. 13 



the nasal fossa of which they may be considered as 

 prolongations. There are four on each side ; viz. : 

 The frontal, the maxillary, the sphenoidal, and the 

 ethmoidal. These cavities are filled with air in 

 their normal state, giving increased volume to the 

 head without increasing its weight. In this manner 

 wide surfaces are furnished for the insertion of mus- 

 cles which in this region are large and numerous. 



The frontal sinus is situated at the inner side of the 

 orbit, presenting very irregular walls which are 

 formed by the frontal, (Fig. 1-13), nasal, (Fig. 17), 

 lachrymal, (Fig. I 8), externally and internally, the 

 ethmoid and superior turbinated bones. 



The frontal sinus communicates freely with the 

 maxillary sinus by a vast opening through the thin 

 bony partition which separates the two sinuses. A 

 vertical bony plate, perforated separates this sinus 

 from that of the opposite side, but is always imper- 

 forate. 



The maxillary sinus formed beneath the orbit, 

 by the superior maxillary, (Fig. 1-4), malar, (Fig. 

 i-n); lachrymal, (Fig. 1-8), externally and inter- 

 nally by the ethmoid and inferior turbinated. This 

 is the largest of the sinuses, and is divided into two 

 compartments by a ridge, which contains the superior 

 dental canal, into an internal small and shallow, and 

 continuous with the sphenoidal, and communicating 

 with the ethmoidal sinus, and an external which is 



