78 



THE SKELETON. 



With inferior turbinated bone. 



Outer surface of right lateral mass 



FIG. 41. Ethmoid bone, 

 (enlarged). 



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 perpendicular plate, which forms part of the septum nasi ; and 

 two lateral masses of cells. 



The Horizontal or Cribriform Plate (Fig. 41) forms part of the anterior fossa 

 of the base of the skull, and is received into the ethmoid notch of the frontal 



bone between the two orbital 

 plates. Projecting upward 

 from the middle line of this 

 plate is a thick, smooth, tri- 

 angular process 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, articu- 

 lates with the frontal bone, 

 and presents two small project- 

 ing alse, which are received 

 into corresponding depressions 

 in the frontal, completing the 

 foramen caecum behind. Its 

 sides are smooth and some- 

 times bulging ; in which case it is found to enclose a small sinus. 1 On each side 

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

 the bulb of the olfactory tract, and perforated by foramina for the passage of the 

 olfactory nerves. 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 cribriform plate, on each side of the crista galli, is a small fissure, which 

 transmits the nasal branch of the ophthalmic nerve ; and at its posterior part a 

 a triangular notch, which receives the ethmoidal spine of the sphenoid. 



The Perpendicular Plate (Fig. 42) 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 circum- 

 ference, and is generally deflected a little to one side. Its anterior border articu- 

 lates with the nasal spine of the frontal bone and crest of the nasal bones. Its 

 posterior border, divided into two parts, articulates by its upper half with the 

 sphenoidal crest of the sphenoid, by its lower half with the vomer. The inferior 

 border serves for the attachment of the triangular cartilage of the nose. On each 

 side of the perpendicular plate numerous grooves and canals are seen, leading from 

 foramina on the cribriform plate; they lodge filaments of the olfactory nerves. 



The Lateral Masses of the ethmoid consist of a number of thin-walled cellular 

 cavities, the ethmoidal cells, interposed between two vertical plates of bone, the 

 outer one of which forms part of the orbit, and the inner one part of the nasal 

 fossa of the corresponding side. In the disarticulated bone many of these 

 cells appear to be broken ; but when the bones are articulated they are closed 

 in at every part, except where they open into the nasal fossae. The upper 

 surface of each lateral mass presents a number of apparently half-broken cel- 



1 Sir George Humphry states that the crista galli is commonly inclined to one side, usually 

 the opposite to that toward which the lower part of the perpendicular plate is bent. The Human 



bkeletoji, 1808, p. 277. 



