THE ETHMOID BONE. 77 



one on each side for the lingula. The six for the anterior sphenoid are one for 

 each lesser wing, two for the anterior part of the body, and one for each sphenoidal 

 turbinated bone. 



Post-sphenoid Division. The first nuclei to appear are those for the greater 

 wings (ali- sphenoids}. They make their appearance between the foramen rotun- 

 dum and foramen ovale about the eighth week, and from them the external ptery- 

 goid plates are also formed. Soon after, the nuclei for the posterior part of the 

 body appear, one on either side of the sella turcica, and become blended together 

 about the middle of ftetal life. About the fourth month the remaining four cen- 

 tres appear, those for the internal pterygoid plates being ossified in membrane 

 and becoming joined to the external pterygoid plate about the sixth month. The 

 centres for the lingulse speedily become joined to the rest of the bone. 



one for each two for anterior 

 lesser wing, part of body. 



i M 



one for each internal--- 



pterygoid plate. ^ \ 



one for f or eacn lingula. \ 



each great wing and external ptery- 

 [goid plate. 

 one for each Sphenoidal turbinated bone. 



FIG. 40. Plan of the development of sphenoid. By fourteen centres. 



Pre-sphenoid Division. The first nuclei to appear are those for the lesser 

 wings (orbito- sphenoids). They make their appearance about the ninth week, at 

 the outer borders of the optic foramina. A second pair of nuclei appear on the 

 inner side of the 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 make their appearance about the fifth month. At birth they 

 consist of small triangular laminae, and it is not till the third year that they 

 become hollowed out and cone-shaped. About the fourth year they become fused 

 with the lateral masses of the ethmoid, and hence, from an embryological point 

 of view, may be regarded as belonging to the ethmoid. 



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 palpebrse, Ob- 

 liquus oculi superior, Superior rectus, Internal rectus, Inferior rectus, External 

 rectus. 



The Ethmoid Bone. 



The Ethmoid (yy^oc, 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 



1 It also sometimes articulates with the tuberosity of the superior maxilla. 



