ix. I OF THE DOG. 105 



mesethmoid cartilage {ME), but the lower part of the pos- 

 terior two-thirds also by the vomer { Vo). The greater part 

 of this bone has the form of a trough, hollow above, em- 

 bracing the inferior, thickened border of the mesethmoid 

 cartilage, and extending a little way behind this so as also 

 to underlie the anterior portion of the presphenoid ; but it 

 also develops from its under surface in the middle line a 

 thin plate, which passes vertically down to the centre of 

 the floor of the nasal passages, and completes the septum 

 inferiorly. 



Above, rather behind the middle of the bone, the lateral 

 plates of the vomer, which embrace the mesethmoid carti- 

 lage, send out sideways a pair of wing-like processes, which 

 join the side walls of the nasal cavity, and form the partial 

 horizontal partition, dividing the narial passage from the 

 olfactory chamber. 



The outer wall of the nasal cavity is formed mainly by 

 four bones : (1) a somewhat quadrate, thin, nearly vertical 

 plate of bone (/V), the pterygoid, attached above to the under 

 surface of the basisphenoid and presphenoid, supported 

 externally by a strong descending process of the alisphe- 

 noid, the external pterygoid plate, ending posteriorly and 

 inferiorly by a free border, and articulating in front with (2) 

 the palatine. This bone {PI) is of much greater extent ; for, 

 besides its vertical portion, forming the outer wall of the 

 nasal canal in front of the pterygoid, it sends from its upper 

 edge a lamina inwards to meet the horizontal plate of the 

 vomer, and aid in forming the roof of the hinder part of the 

 narial passage. It also sends a strong horizontal lamina 

 inwards from its lower edge, which, meeting its fellow in the 

 middle line, forms the posterior part of the floor of the narial 

 passage. In addition to these it sends a broad plate upwards 



K forwards in the inner wall of the orbit. 



