THE NASAL CAVITIES. 447 



presents very irregular walls, which are formed by the frontal, nasal, lachry- 

 mal, and ethmoidal bones, and the superior portion of the ethmoidal 

 turbinated bone. It communicates with the superior maxillary sinus by 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 orbit, between the 

 maxillary, zygomatic, ethmoid, and lachrymal bones, this diverticulum is the 

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

 canal, which traverses it. The internal compartment constitutes a kind of 

 shallow cavity, continuous with the sphenoidal sinus, and presents a narrow 

 slit, which penetrates to the ethmoidal sinus. The external compartment 

 is separated, in front, from the maxillary sinus by a partition which M. 

 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 mucous layers laid against each other. This 

 compartment is prolonged backwards into the maxillary protuberance, and 

 the roots of the two last molars project into its anterior. 



SPHENOIDAL SINUS. This is the smallest, after that ot the great 

 ethmoidal cell. Formed by the sphenoid and palate 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 ; the other posterior, hollowed 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. By this name is designated the internal cavity of the 

 large ethmoidal cell, which constitutes a real sinus, and which a narrow 

 slit brings into communication with the superior maxillary sinus. 



INFERIOR MAXILLARY SINUS. This last diverticulum is remarkable because 

 of its not communicating with the others. 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 twe compart- 

 ments : an internal, prolonged into the superior cavity of the maxillary 

 turbinated bones ; and an external, the smallest, showing the roots of the 

 fourth molar, rarely those of the third. It does not descend, as Rigot has 

 asserted, above the three front molars ; but 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 trepan, in order to arrive 

 at its interior. 



COMMUNICATING ORIFICE OF THE SINUSES WITH THE NASAL FOSSA. All 

 the sinuses of one side communicate with the corresponding nasal fossa by 

 the curved slit which has been observed at the bottom of the middle meatus. 

 This slit penetrates the superior maxillary sinus, under the septum that 

 separates it from the frontal sinus ; it also enters the inferior maxillary 

 sinus, which thus communicates solely with the nasal cavity, while the 

 other diverticuli open in common into this cavity through the medium of 

 the superior maxillary sinus. 



Mucous MEMBRANE OF THE SINUSES. In entering the sinuses to cover 

 their walls, the pituitary membrane becomes extremely thin, and loses its 

 great vascularity ; it is applied immediately to the bones, and serves as a 

 periosteum. 



DEVELOPMENT OF THE SINUSES. These cavities begin to be developed in 



