76 OS ETHMOIDES. 



The cribriform plate is oblong in shape, and firm in Its 

 structure ; in the middle of the anterior extremity the crista 

 gain projects from its upper surface, dividing it into two lateral 

 portions, each of which is rather concave, and occupied by the 

 bulbous extremity of the olfactory nerve ; it is perforated by 

 many foramina, which transmit the fibres of the aforesaid 

 nerve. Near the crista galli, on each side, there is a small fis- 

 sure, through which passes a nervous filament derived from the 

 ophthalmic brancli of the fifth pair. The crista galli varies in size 

 in different subjects : the beginning of the falciform process of 

 the dura mater is attached to it, and with the opposite part of 

 the OS frontis it forms the foramen caecum, already mentioned. 

 It is very conspicuous in the basis of the cranium. 



The nasal plate of the ethmoid bone seems to be continued 

 downwards from the crista galli through the cribriform plate. It 

 is thin, but firm ; it forms the upper portion of the septum of 

 the nose, and, to complete the partition, it unites with the vomer 

 and with a plate of cartilage before. It is very often inclined 

 to one side, so that the nostrils are not of equal size. 



At a small distance from this perpendicular plate, on each 

 side of it, the cellular portions originate from the lower surface 

 of the cribriform plates ; they extend from before backward, 

 and are as long as the ethmoid bone ; their breadth between 

 the eye and the cavity of the nose varies In difl'erent subjects, 

 from half an inch to more ; they extend downwards from the 

 root of the nose or from the cribriform plate, more than half 

 way to the roof of the mouth. Their external surface on each 

 side forms a part of the surface of the orbit of the eye, and Is 

 called OS planum ; their internal surface forms part of the 

 external lateral surface of each nostril. This surface extends 

 the whole depth of the nostril, from before backward ; but in 

 many skeletons it is extremely imperfect, owing to the great 

 brlttleness of the bony plates of which it is composed. When 

 the bone is perfect, the uppermost half part of this internal 

 surface is uniformly flat, and rather rough ; but below it, about 

 the middle of the bone, a deep groove begins, which extends 

 downwards and backwards, to the posterior extremity ; this is 



