THE ETHMOID BOXE. 



185 



foramina shortly after, and. becoming united, form the front part of the body of 

 the bone. The remaining two centres for the sphenoidal turbinated bones do not 

 make their appearance until the end of the third year. 



The pre-sphenoid is united to the body of the post-sphenoid about the eighth 

 month, so that at birth the bone consists of three pieces viz. the body in the 

 centre, and on each side the great wings with the pterygoid processes. The lesser 

 wings become joined to the body at about the time of birth. At the first year 

 after birth the greater wings and body are united. From the tenth to the twelfth 

 year the spongy bones are partially united to the sphenoid, their junction being 

 complete by the twentieth year. Lastly, the sphenoid joins the occipital from the 

 eighteenth to the twenty-fifth year. 



Articulations. The sphenoid articulates with all the bones of the cranium, 

 and five of the face the two malar, two palate, and vomer : the exact extent of 

 articulation with each bone is shown in the accompanying figures. 1 



Attachment of Muscles. To eleven pairs : the Temporal. External pterygoid, 

 Internal pterygoid. Superior constrictor, Tensor palati, Levator palpebrae, Ob- 

 liqutis oculi superior, Superior rectus. Internal rectus, Inferior rectus, External 

 rectos. 



The Ethmoid Bone. 



The Ethmoid (-/fluo;. 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 perpendicular plate, which forms part of the septum nasi ; and 

 two lateral masses of cells. 



The Horizontal or Cribriform Plate (Fig. 147) 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 



i. yUL 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 alae. 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. 2 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 



1 It also sometimes articulates with the tuberosity of the superior maxilla (see p. 190). 



* Sir George Humphry state* 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 Skde- 



S, p. 277.) 



With inferior turbinated bone. 



FIG. 147. Ethmoid bone. Outer surface of right lateral mass 

 (enlarged). 



