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) 1s bounded by the nasal bones and the 
premaxillie. 
The posterior extremity or fundus is separ- 
ated from the cranial cavity by the cribriform 
plate of the ethmoid, and is largely occupied by 
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 paranasales), 
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 line 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. 
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- 
Fic. 58.—SKuULL or Youna Horsr} Dorsat 
View, witH Sinuses OpeNepD Up. 
1, 2, Ends of frontal sinus; 3, fronto- 
The last three cheek teeth project 
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’; 8, 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. 
even as far forward as the anterior end of the facial crest. 
In the recent state, 7. e., when covered by the mucous membrane on 
orbit than is stated above. 
tum is commonly about two inches (ca. 5 em.) 
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. 
The position and shape of the septum are very 
variable. It is often further forward—in some cases 
Exceptionally it is much nearer the 
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 
