ETHMOID BONE. 



77 



Fig. 38. Ethmoid Bone. Outer Surface of Right Lateral 

 Mass (enlarged). 



THE ETHMOID BOXE. 



The Ethmoid (i?9u6j, a sieve) is an exceedingly light spongy bone, of a cubical 

 form, situated at the anterior part of the base of the cranium, between the two 

 orbits, at the root of the nose, 

 and contributing to form each 

 of these cavities. It consists 

 of three parts : a horizontal 

 plate, which forms part of the 

 base of the cranium ; a per- 

 pendicular plate, which forms 

 part of the septum nasi ; and 

 two lateral masses of cells. 



The Horizontal or Cribri- 

 form Plate (fig. 38) forms 

 part of the anterior fossa of 

 the base of the skull, and is 

 received into the ethmoid 

 notch of the frontal bone be- 

 twBen the two orbital plates. 

 Projecting upwards from the 

 middle line of this plate, is a 

 thick smooth triangular pro- 

 cess of bone, the crista galli, 

 so called from its resemblance 

 to a cock's comb. Its base joins the cribriform plate. Its posterior border, long, 

 thin, and slightly curved, serves for the attachment of the falx cerebri. Its 

 anterior border, short and thick, articulates with the frontal bone, and presents 

 two small projecting also, which are received into corresponding depressions in 

 the frontal, completing the foramen cascum behind. Its sides are smooth, and 

 sometimes bulging, when it is found to inclose a small sinus. On each side of 

 the crista galli, the cribriform plate is narrow, and deeply grooved, to support 

 the bulb of the olfactory nerves, and is perforated by foramina for the passage of 

 its filaments. These foramina are arranged in three rows ; the innermost, which 

 are the largest and least numerous, are lost in grooves on the upper part of the 

 septum; the foramina of the outer row are continued on to the surface of the upper 

 spongy bone. The foramina of the middle row are the smallest ; they perforate the 

 bone, and transmit nerves to the roof of the nose. At the front part of the cribri- 

 form plate, on each side 



Shown by 



of the crista galli, is a 

 small fissure, which 

 transmits the nasal 

 branch of the ophthal- 

 mic nerve; and at its 

 posterior part a trian- 

 gular notch, which re- 

 ceives the ethmoidal 

 spine of the sphenoid. 



The Perpendicular 

 Plate (fig. 89) is a thin 

 flattened lamella of 

 bone, which descends 

 from the under surface 

 of the cribriform plate, 

 and assists in forming 

 the septum of the nose. 

 It is much thinner in 

 the middle than at the 



Fig. 39. Perpendicular Plate of Ethmoid (enlarged), 

 removing the Right Lateral Mass. 



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