152 TOPOGRAPHICAL ANATOMY OF THE 



separation of the two compact plates that, in all the bones of the skull, 

 are separated by a stratum of looser bone. The sinuses communicate 

 with the nasal cavity, either directly or indirectly. They, therefore, 

 contain air and are lined by a thin, mucous membrane continuous with 

 that of the nose. Such cavities, or cells, occur in the ethmoid bone ; 

 but the most important sinuses are the maxillary, coucho-frontal and 

 spheuo-palatine. Before beginning the study of these in the head upon 

 which the dissection is now being conducted, it is useful to plot out 

 their limits upon an intact macerated skull. 



The maxillary sinus (sinus maxillaris) is bounded by the maxilla, 

 zygomatic, lachrymal, ethmoid, frontal and ventral turbinated bones. 

 The upper limit of the sinus may be defined by drawing a line from the 

 medial angle of the eye (lachrymal fossa in the macerated skull) to the 

 lower angle of the nostril (the junction of the lower and middle thirds 

 of the nasal process of the incisive bone in the macerated specimen). 

 The line should have a slight curve, with the convexity upwards, and it 

 should be remembered that it indicates roughly the position of the naso- 

 lachrymal duct. Posteriorly the sinus extends into the raaxillar}' tuber, 

 that is, to the level of the lateral angle of the eye. Its anterior limit is 

 variable, but, in the majority of horses, may be regarded as occurring 

 about 5 cm. in front of the end of the facial crest. 



The maxillary sinus is divided into anterior and posterior parts by a 

 complete septum, the position of which is variable, as is also the angle 

 that it forms with the transverse plane. In all horses it slopes obliquely 

 downwards and forwards, and its average position may be indicated by 

 saying that it coincides with an oblique transverse plane cutting the 

 face midway between the margin of the orbit and the end of the facial crest. 



If now the sinus be opened by the piecemeal removal of its lateral 

 wall, its exact extent may be determined. The dissector will observe 

 that the infraorbital canal traverses both parts of the sinus, and is 

 connected with the ventral wall or floor of the cavity by a plate of bone. 

 Thus it comes to pass that the sinus is imperfectly divided into medial 

 and lateral compartments, communication between which is over the 

 infraorbital canal. For surgical reasons it is important to remember, 

 moreover, that the roots of the last three or four cheek-teeth are 

 associated with the lateral compartment of the sinus, a thin, irregular 

 layer of bone being all that separates them from the interior of the 

 cavity. The septum between the anterior and posterior parts of the 

 sinus occurs, in the average animal, on a level with the second last 

 tooth. 



The dimensions of the sinus vary in different individuals : they also 

 undergo very marked changes in the same individual at different ages. 



