190 ANATOMY OF THE HEAD. 



of quadrupeds is very differently formed from our own. In 

 the horse, the nose constitutes a very considerable portion 

 of the head ; having two exterior communications with the 

 air, called nostrils, and an interior opening to the fauces. 

 The openings of the nostrils are limited anteriorly by the 

 nasal and intermaxillary bones. Immediately above the 

 arch of the palate, which is situated interiorly, the nose 

 communicates with the frontal sinuses, by a species of flap 

 or valve ; through which a probe can be easily thrust, but 

 which must admit but little air into the sinuses. The nose 

 is divided in the middle by the septum narium {Plate VI. a) ; 

 which is cartilaginous ; and rests in a bone named the vomer, 

 that is grooved for its reception. 



The frontal sinuses {Plate VI. ; Plate VII. d, d) are 

 formed by the separation of the two tables of the frontal 

 bones. There is usually a bony partition which forms them 

 into two equal portions, and other bony prolongations are 

 seen still further subdividing these spaces : these sinuses 

 communicate superiorly with the nasal cavities, and are lined 

 by a continuation of the same membrane. The frontal 

 sinuses freely communicate with the nasal and the maxillary ; 

 each of which are thus named because formed by divisions 

 of those bones. 



The highly vascular and sensitive expansion called the 

 pituitary memhrane lines the whole nasal fossae throughout 

 all their compartments. It is a portion of the great mucous 

 track, which begins at the eyes, nose, and mouth ; extends 

 to the lungs, stomach, and intestines, and terminates at the 

 anus. It is furnished with a mucus secreted throughout 

 the whole extent of its surface; whereby the surface is always 

 kept pliant, soft, and susceptible ; and it is reflected over the 

 superior and inferior turbinated bones {Plate VII. a, h), 

 as well as the cartilaginous septum {Plate VI. a) ; by this 

 mucus likewise insects are prevented from penetrating into 

 the lungs. The evident use of the pituitary membrane ap- 

 pears to be a medium for the expansion of the olfactory 

 nerves, whereby the effluvia of different substances are 

 ascertained. 



Ductus ad nasum. — The puncta lachrymalia are the open- 

 ings to a canal within the lachrymal bone, called the nasal 

 duct; which is continued from the eye, between the turbinated 



