THE BONES OF THE HEAD 



93 



and it here sometimes completes the foramen caecmn. 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 Galli 

 Nasal Groove. ' 



Groove for Anterior 

 Ethmoidal Canal 



, Vertical Plate 

 -Right Alar Process 



nt. Ethmoidal Cells 



Infiindibulum 



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, whuj 

 transmits the nasal branch of the ophthalmic nerve and nasal li^tUS. 

 of the anterior ethmoidal artery to the nasal fossa, ^{a, and 

 backwards and outwards from this slit to the anterior, 

 ~,s^oove on the upper border of the lateral mass is the Tj- margin of 

 the 01-L.l.the nasal nerve. The posterior border of t depressions 

 upon it, and coji.iciuj tifel ethmoidal spine of the si these depres- 

 sions this border presents two trknsvelsc' ^^'"nds doibout half an 

 inch apart, which, with corresponding grooves' on the orbital 

 plate of the frontal, form the anterior and posterior ethmoidal or 

 internal orbital canals. These open upon the inner wall of the 

 orbit, and the antctior transmits the anterior ethmoidal vessels 

 and the nasal nerve, whilst the posiertor gives passage to the 

 posterior ethmoidal vessels and the spheno'-ethmoidal nerve. The 

 inferior border, which is free on the outer wall of the nasal 



