200 DISSECTION OF THE DOG 



narrow in front but becomes more spacious behind. These three passages are 

 put into communication with each other by the common meatus (meatus nasi 

 communis), a cleft-like channel intervening between the turbinated bones 

 and the nasal septum. 



As was observed during the removal of the septum, the posterior part of the 

 cavity of the nose is divided into two parts by the transverse lamina of the 

 ethmoid which joins the vomer. The dorsal and more extensive portion is 

 olfactory in function, the lower is respiratory. 



The most anterior part of the nasal cavity, i.e. that part corresponding 

 to the moveable septum, forms the vestibule of the nose (vestibulum nasi) and 

 contains two mucous folds continuous with the two turbinated bones. The 

 dorsal fold as such soon disappears, and its place may be said to be taken by 

 two short, oblique folds. The ventral fold expands anteriorly and ends abruptly. 

 Close to the floor of the vestibule, and on a level with the end of the ventral 

 mucous fold, is the opening of the naso-lachrymal duct (ductus naso-lacrimalis) 

 by which the tears reach the nose. 



Organon vomeronasale [Jacobsoni]. — If a fine probe be introduced 

 into a small opening at the side of the incisor papilla, it will traverse the naso- 

 palatine duct and appear in the floor of the nasal cavity close to the septum 

 and on a level with the canine tooth. Into the naso-palatine duct opens 

 the tubular vomero-nasal organ of Jacobson. The organ is enclosed in the 

 vomero-nasal cartilage (cartilago vomero-nasale) situated along the base of 

 the nasal septum, and is innervated by the olfactory and the nasal branch of 

 the spheno-palatine nerve. 



Sinus Maxillaris. — It is best to open the maxillary sinus from the outside. 

 As a preliminary step the infraorbital canal should be opened up and cleared 

 of its contents. The inner wall of the canal is formed by a thin plate of bone 

 which may be fairly easily removed without injury to the underlying mucous 

 lining of the sinus. The bony opening may be enlarged as much as desired 

 and the mucous membrane should then be incised to such an extent as to allow 

 of a good view of the interior. 



The maxillary sinus has an oval outline and coincides in extent with the 

 last four maxillary teeth. It will be noted that the communication between 

 the sinus and the nasal cavity (aditus naso-maxillaris) is large and oval, but is 

 not in the lowest part of the sinus. The aditus opens into the nasal cavity 

 close to where the ventral limb of the middle meatus joins the inferior meatus. 



Sinus frontalis. — When the skull was sawn longitudinally one of the 

 frontal sinuses was opened. The dimensions of the sinus should be ascertained. 

 It is generally of good size, and more or less perfectly divided into two or more 

 parts, but in this respect much depends upon the breed of the dog which is 

 being dissected. The sinus opens into the ethmoidal meatus of the nasal cavity. 



If the mucous membrane covering the lateral wall of the nasal cavity be 

 examined it -will be found that a certain zone of the lining of the vestibule is 



