DISSECTION OF THE DOG 199 



The cartilaginous septum (septum nasi cartilagineum) fits between the two 

 plates of the vomer, and fills the interval between this bone, the perpendicular 

 plate of the ethmoid and the nasal bones. As already stated, the cartilaginous 

 septum is continued forwards above the incisive bones, where it is more or less 

 moveable (septum mobile nasi). 



Dissection. — By the exercise of a reasonable amount of care, the septum 

 may be removed bit by bit in such a way that the mucous membrane 

 clothing its opposite side is left intact. This permits the dissector to 

 examine the blood-vessels and nerves of the partition. In removing the 

 posterior part of the septum a horizontal plate of bone, the transverse 

 lamina of the ethmoid, will be observed to cut the nasal cavity into 

 upper and lower divisions. 



In common with that of the nasal cavity generally, the mucous mem- 

 brane covering the septum is richly suppUed with both vessels and nerves. 

 The vessels are derived from the ethmoidal branch of the ophthalmic artery 

 which ramifies over the upper area of the septum, and the nasal branch of the 

 spheno-palatine artery which supplies the lower region. 



The nerves are derived from two sources : (1) The olfactory nerves are 

 distributed over the dorsal and posterior part of the septum and also supply 

 the vomero-nasal organ (of Jacobson) ; (2) the trigeminal nerve contributes 

 branches from both its ophthalmic and its maxillary divisions. The ethmoidal 

 branch of the ophthalmic nerve terminates over the dorsal part of the septum, 

 and the nasal branch of the spheno-palatine nerve supplies the lower part. 



Dissection. — With a pair of scissors cut along the upper border of the septal 

 mucous membrane, and so obtain a view of the interior of the nasal 

 cavity. 



Cavtjm nasi. — Each nasal cavity is narrow and elongated, but its relative 

 length in the dog is largely dependent upon the breed. The narrow dorsal 

 boundary or roof of the cavity is formed by the nasal and frontal bones, while 

 the palatine, maxillary, and incisive bones enter into the composition of its 

 floor. The nasal septum constitutes the medial wall. The lateral wall is 

 very uneven as the result of the projection of the turbinated bones and the 

 ethmoidal cells into the cavity. The dorsal turbinated bone (concha nasalis 

 superior) is relatively much narrower and has a smoother surface than the 

 ventral turbinated bone (concha nasaUs inferior). The ethmoidal cells form 

 an elongated triangular mass insinuated between the posterior ends of the two 

 turbinated bones. 



The intrusion of the turbinated bones divides the nasal cavity into passages, 

 the meatus nasi. The upper meatus (meatus nasi superior) is narrow and 

 shallow ; the middle meatus (meatus nasi medius) is short and narrow and 

 divides posteriorly into two parts ; and the lower meatus (meatus nasi inferior) is 



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