OSTEOLOGY OF THE HORSE. 47 



rymal bones, whose orbital processes constitute a thin partition be- 

 tween it and the orbit ; the inferior parts consist of the excavations 

 in the superior maxillary bone; superiorly, the sinus is open, being 

 there continuous with the frontal : the boundary line between 

 these cavities is marked by the suture uniting the lachrymal to the 

 frontal and nasal bones on the outer side, and by the prominent 

 crest formed by the junction of the superior turbinated with the 

 ethmoid bone on the inner ; underneath which part, through a 

 curved (and in the recent subject sort of valvular) fissure, the sinus 

 opens into the middle meatus, between the bases of the turbinated 

 bones. The cavity is but small, and still more irregular, in the 

 young subject, in consequence of the intrusion of the yet uncut 

 molar teeth. 



The frontal sinus, then, terminates in the nasal, but both dis- 

 charge themselves into the maxillary ; the maxillary has also a 

 blind termination, but empties itself into the posterior part of the 

 middle nasal meatus. 



The sphenoidal sinus is situated within the palatine por- 

 tion of the body of the sphenoid bone. It has no existence in 

 the young subject, the bone being solid throughout; but in pro- 

 cess of growth a cavernous hollow is formed, which, from the 

 secession and attenuation of the laminae of the bone, continues to 

 enlarge. It communicates, by two ovoid openings, with the 

 ethmoidal sinuses. 



The ethmoidal sinuses are two cavities, separated by 

 the perpendicular plate, situated beneath the ethmoidal cells. 

 They have openings in front, communicating with the lowermost 

 and largest grooves of the same bone, and with the palatine 

 sinuses. 



The palatine sinuses are formed between the superior 

 maxillary and palate bones; are situated below and in front of 

 the former; are separated from each other by the vomer ; and 

 open into the maxillary sinuses : they are irregular in form and 

 cavernous interiorly. They are not to be found in the young sub- 

 ject. Some might be inclined to treat them as parts of the max- 

 illary sinuses; they are, however, as perfectly distinct from the 

 latter as the frontal are. 



IV. — the mouth. 



The mouth is the cavity included between the superior and infe- 

 rior maxillae, making (in the skeleton)onecommon vacuity with the 

 inter-maxillary space. Its antero-posterior dimensions can be but 

 little varied ; but its supero-inferior diameter will be increased in 

 the ratio of the distance to which the inferior maxilla recedes from 

 the superior; the cavity during the distraction of the jaws as- 



