ix.] OF THE DOG. 129 



its anterior end is free, and forms the upper boundary of 

 the anterior nares ; its outer side is in contact with the 

 frontal, maxilla, and premaxilla ; and its straight inner edge 

 lies against that of the corresponding bone of the opposite 

 side. 



Within each nasal cavity are two very singular bones, 

 each being composed of a number of delicate lamellae 

 folded and arranged in an exceedingly complex manner, 

 forming a mass with so many passages and perforations that 

 the term " spongy bones " has been applied to them. 



The most posterior is the larger, and placed rather higher 

 than the other, its anterior extremity (ET) overlapping it. 

 It completely fills the proper olfactory chamber ; its hinder 

 extremity occupying the gap left in the cranial wall in front 

 of the anterior segment of the brain-case. The various 

 laminae are all connected together and to the hinder end of 

 the mesethmoid, by a plate of bone (CE) so full of perfora- 

 tions of varied form and size that it is called the cribriform 

 plate, and from it the name of ethmoid (or sieve-like) is 

 commonly applied to all the bony structures with which 

 it is united. On their outer side these laminae are con- 

 nected to a thin flat plate of bone (the so-called os planuni) 

 which lies against the inner wall of the maxillary, but does 

 not ordinarily contract any union with it. 



This bone results from the ossification of the complexly 

 folded cartilage, over the surface of which the olfactory 

 nerves are spread, the division into laminae permitting a 

 great increase of sensitive surface. As, although originally 

 distinct, it subsequently unites with the mesethmoid, by 

 means of the cribriform plate, it is considered in human 

 anatomy as part of the same bone, and described under 

 the name of "lateral mass of the ethmoid," being further 

 divided into the superior and middle " turbinated bones " ; 



K 



