350 
ANATOMY OF THE HORSE. 
opposite to the iinaginaiy line above alluded to.. The cavity 
is traversed and divided into several unequal open apartments 
and recesses by septa; the principal of which is one extended be¬ 
tween the superior and inferior sides: it is partitioned from the 
opposite sinus by the nasal spine. The sinus is but small in the 
young compared to its proportionate dimensions in the adult sub¬ 
ject: it continues to increase afterwards with age, and at length 
extends throughout the whole of the frontal bone. 
The nasal sinuses, formed by the nasal bones above and 
the superior turbinated bones behind, are nothing more than the 
cul-de-sac or blind terminations of the frontal sinuses. 
The maxillary sinuses, the largest of these cavities, are 
spacious but very irregularly formed. They are situated below 
and in front of the frontal. Of this sinus, on either side, the 
posterior and external walls are formed by the malar and lach¬ 
rymal bones, whose orbital processes constitute a thin pailition 
between it and the orbit ,• the inferior parts are 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 j unction of the superior turbinated ethmoid 
with the bone on the inner; underneath which, through a curved 
(and in the recent subject sort of valvular) fissure, this sinus opens 
into the middle meatus, between the bases of the turbinated bones. 
This 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 maxillaiy; the maxillaiy has also a 
blind termination, but empties itself into the posterior end 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¬ 
gress 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, and situated beneath the ethmoidal cells. 
They have openings in front, communicating with the lowermost 
and largest grooves of the same bone, and witli the palatine 
sinuses. 
The palatine sinuses arc formed between the superior 
