THE NASAL CAVITIES. 525 



First Group. 



Frontal Sinus. — This cavity, situated at the inner side of the orbit, presents 

 very irregular walls, which are formed by the frontal nasal, lachrymal, and 

 ethmoid bones, and the superior portion of the ethmoidal turbinated bone. It 

 communicates with the superior maxillary sinus l;)y a vast opening, made in a 

 very thin bony partition. A thick vertical plate, often bent to the right or left, 

 but always imperforate, separates this sinus from that of the opposite side. 



Superior Maxillary Sinus. — Channeled beneath the orlnt, between the 

 maxillaiy, malar, ethmoid, and lachrymal bones, this diverticulum is the 

 largest of all, and is divided into two great compartments by the maxillo-dental 

 canal, which traverees it. The internal compartment is a kind of shallow cavity, 

 continuous with the sphenoidal sinus, and presents a narrow slit, which opens 

 into the etlimoidal sinus. The external compartment is separated, in front, 

 from the maxillary sinus, by a partition which Goubaux has, contrary to the 

 generally received opinion, demonstrated to be imperforate at all periods of life ; 

 though he has sometimes found it so thin as only to consist of two layers of 

 mucous membrane laid against each other. This compartment is prolonged 

 backwards into the maxillary protuberance, and the roots of the two last molare 

 project into its interior. 



Sphenoidal Sinus. — This is the smallest, after that of the great ethmoidal 

 cell. Formed by the sphenoid and palatine bones, this cavity is very irregular, 

 and is subdivided by incomplete septa into several compartments, which may be 

 always reduced to two — an anterior, comprised between the palatine laminae ; and 

 a posterior, in the body of the sphenoid bone. In contact, on the median line, 

 with the sinus of the opposite side, it is separated from it by a twisted plate, 

 which is constantly perforated, even in young animals. 



Ethmoidal Sinus. — This is a cavity in the large ethmoidal cell ; it consti- 

 tutes a real sinus, and has a narrow slit which brings it into communication with 

 the superior maxillary sinus. 



Second Group. 



Inferior Maxillary Sinus. — This last diverticulum is remarkable, because 

 of its not communicating with the otiiers. Excavated in the supermaxillary 

 bone, and separated from the superior sinus by the imperforate septum previously 

 mentioned, it is divided, like the latter cavity, into two compartments — an 

 internal, prolonged into the superior cavity of the supermaxillary turbinated 

 bones ; and an external — the smallest — showing the roots of the fourth molar, 

 rarely those of the third. It does not descend, as Eigot has asserted, above the 

 three anterior molars ; supposing the head to be vertical, it does not extend, in 

 the adult Horse, beyond the extremity of the maxillary ridge, in front of which it 

 would be necessary to trephine, in order to reach it. It is sometimes larger on 

 one side than the other. 



The inferior maxillary sinus, in the Ass, communicates with the superior 

 maxillary sinus. " These two parts of the maxillary sinus, or the two maxillary 

 sinuses," says CTOubaux, "always communicate largely with each other in the 

 Ass. I have never seen an exception to this ; and the peculiarity is due to the 

 fact, that the base or posterior extremity of the inferior turbinated bone is checked 

 in its development, and does not curve sufficiently to join the inner surface of 

 the supermaxilla,*nd become attached thereto." 

 36 



