THE BONES OF THE HEAD 



93 



and it here sometimes completes the foramen caecum. The 

 posterior border is prolonged backwards as a median ridge, and 

 on either side of this ridge and the crista galli is the olfactory 

 groove, which lodges the olfactory tract and bulb. Each half of 

 the cribriform plate, which lies in the roof of the corresponding 

 nasal fossa, is pierced by foramina for the filaments of the olfactory 

 bulb. The foramina in each half are arranged in three sets, as 

 follows — a middle set, which are simple perforations, and an internal 

 and external set, which lead into small canals. These canals 

 descend on the perpendicular plate and inner surface of the lateral 

 mass respectively, branching and opening out as they descend. 

 All the foramina lead to the upper part of the corresponding nasal 

 fossa. At the anterior and inner part of each half of the cribriform 

 plate, close to the side of the crista galli, near its anterior border, 



Crista Gall 

 Nasal Groove., 



Groove for Anterior 

 Ethmoidal Canal 



, Vertical Plate 

 -Right Max Process 



Ant. Ethmoidal Cells 



Infundibulum 



Orbital Plate i Olfactory Groove on Cribriform Plate 



Groove for Post. Ethmoidal Canal 



Fig. 57. — The Ethmoid Bone (Superior View). 



there is an antero-posterior fissure, called the nasal slit, whicK 

 transmits the nasal branch of the ophthalmic nerve and nasal branch 

 of the anterior ethmoidal artery to the nasal fossa. Leading 

 backwards and outwards from this slit to the anterior, ethmoidal 

 groove on the upper border of the lateral mass is the nasal groove, 

 also foi the nasal nerve. The posterior border of the cribriform 

 plate articulate^ ..Icii the ethmoidal spine of the sphenoid. 



The perpendicular plate (meseLamoid) extends downwards from 

 the cribriform plate in the middle line. It lies between the lateral 

 masses, where it forms about the upper third of the nasal septum, 

 and it is usually inclined more to one side than the other. It is 

 very thin and irregularly quadrilateral. The superior border projects 

 above the cribriform plate and forms the crista galli. The inferior 

 border articulates, in front, with the septal cartilage of the nose, 

 and behind, with the alae of the vomer in the intervening cleft. 



