OS ETHMOIDES. 75 



is somewhat excavated, and forms a large superficial groove for 

 the medulla oblongata ; on each side of this groove is a small 

 furrow for the inferior petrous sinuses. 



The two upper edges of the occipital bone are serrated, to 

 articulate, with those of the parietal, and form the lambdoidal 

 suture. The inferior edges are divided into two portions by a 

 small prominence called the jugular eminence ; the upper and 

 posterior portion is also serrated for articulation with the 

 mastoid portion of the temporal ; the inferior portion, which 

 is not serrated, applies to the petrous portion of the temporal 

 bone, and a notch in it contributes to the formation of the fora- 

 men lacerum. 



The upper angle of this bone is acute, the lateral angles are 

 obtuse, and the inferior truncated. It is articulated with the 

 parietal, the temporal, and the sphenoidal bones. 



Os Ethmoides. 



The 05 ethmoides is truly one of the most curious bones of 

 the human body. It appears almost a cube, not of solid bone, 

 but exceedingly light and spongy, and consisting of many con- 

 voluted plates, which form a network like honey-comb. It is 

 firmly enclosed in the os frontis, betwixt the orbitary processes 

 of that bone. One horizontal plate receives the olfactory nerves, 

 which perforate that plate with such a number of small holes, 

 that it resembles a sieve ; whence the bone is named cribriform, 

 or ethmoid. " Other plates are so arranged that they form a 

 cellular structure, on which the olfactory nerves are expanded 

 by means of a particular membrane ; while an additional plate, 

 appropriated to the nose, descends into that cavity in a perpen- 

 dicular direction, and forms a large proportion of the partition 

 which divides it into two chambers. 



The cribriform plate is situated in the anterior part of the 

 basis of the cranium. The cellular part occupies most of the 

 space between the orbits of the eyes, and the perpendicular plate 

 is to be found in the septum of the nose. 



The ethmoid bone, for the purposes of description, may be , 

 divided into three parts, viz. the cribriform plate, the nasal or ( 

 perpendicular lamella, and the cellular portions. , 



