84 



THE SKELETON OF THE HORSE 



overhung by the ventral turbinate bone; it is much the largest and is the direct 

 path between the nostrils and posterior nares. 



The osseous nasal aperture (Apertura nasi 

 ossea) is bounded by the nasal bones and the 

 |f -'.-W premaxillffi. 



The posterior extremity or fundus is separ- 

 ated from the cranial cavity by the cribriform 

 plate of the ethmoid, and is largely occupied by 

 fc 9&V the lateral masses of that bone. 



THE PARANASAL SINUSES 



Connected directly or indirectly with the 

 nasal cavity, of which they are diverticula, are 

 four pairs of air-sinuses (Sinus paranasals), 

 viz., maxillary, frontal, sphenopalatine, and 

 ethmoidal. 



The maxillary sinus (Sinus maxillaris) is the 

 largest. Its lateral wall is formed by the maxilla, 

 the lacrimal, and the malar. It is bounded medi- 

 ally by the maxilla, the ventral turbinate, and the 

 lateral mass of the ethmoid bone. It extends 

 backward to a transverse plane in front of the 

 root of the supraorbital process, and its anterior 

 limit is indicated approximately by a hne drawn 

 from the anterior end of the facial crest to the 

 infraorbital foramen. Its dorsal boundary cor- 

 responds to a line drawn backward from the in- 

 fraorbital foramen parallel to the facial crest. 

 The ventral wall or floor is formed by the alve- 

 olar part of the maxilla; it is very irregular and 

 is crossed by bony plates running in various 

 directions. The last three cheek teeth project 

 up into the cavity to an extent which varies 

 with age; they are covered by a thin plate of 

 bone. The cavity is divided into anterior and 

 posterior parts by an oblique septum (Septum 

 sinus maxillaris). The lateral margin of the sep- 

 tum is commonly about two inches (ca. 5 cm.) 

 from the anterior end of the facial crest; from 

 here it is directed inward, backward, and up- 

 ward. The upper part of the septum (formed 

 by the posterior end of the ventral turbinate 

 bone) is very delicate and usually cribriform. 



FiQ. 58. — .Skull of Young House; Dorsal 

 View, with Sinuses Opened Up. 

 1, 2, Ends of frontal sinus; 3, fronto- 

 maxillary opening: 4, dorsal turbinate bone; 

 5, lateral mass of ethmoid bone; 6, 6', two 

 compartments of maxillary sinus; 7, septum 

 between 6 and 6'; S, orbit; 9, anterior end 

 of facial crest; A, frontal bone; B, nasal 

 bone; C, lacrimal bone; D, maxilla. Note 

 difference in position and form of septum in 

 maxillary sinus as compared with preceding 

 figure. 



The position and shape of the septum are very 

 variable. It is often further forward — in some cases 

 even as far forward as . the anterior end of the facial crest. Exceptionally it is much nearer the 

 orbit than is stated above. In the recent state, i. e., when covered by the mucous membrane on 

 both surfaces, it is nearly always complete, but in very exceptional cases there is an opening of 

 variable size in the upper part. 



The anterior compartment, often called the inferior maxillary sinus, is partially 

 divided by the infraorbital canal into a lateral maxillary part and a medial smaller 

 turbinate part. The latter communicates with the middle meatus by a very narrow 

 slit situated at its highest part. The posterior compartment, often called the 



