THE ETHMOID BONE. 187 



face is rough and marked above by numerous grooves, which run nearly verti- 

 cally downward from the cribriform 

 plate : they lodge branches of the 

 olfactory nerve, which are distributed 

 on the mucous membrane covering the 

 bone. The back part of this surface 

 is subdivided by a narrow oblique 

 fissure, the superior meatus of the 

 nose, bounded above by a thin, curved 

 plate of bone, the superior turbinated 

 bone. By means of an orifice at the 

 upper part of this fissure the posterior 

 ethmoidal cells open into the nose. 

 Below, and in front of the superior 

 meatus, is seen the convex surface of u*Sin!'(3Kf. ^ Inner 8UrfaCe f right 

 the middle turbinated bone. It extends 



along the whole length of the inner surface of each lateral mass ; its lower mar- 

 gin is free and thick, and its concavity, directed outward, assists in forming the 

 middle meatus. It is by a large orifice at the upper and front part of the middle 

 meatus that the anterior ethmoidal cells, and through them the frontal sinuses, 

 communicate with the nose by means of a funnel-shaped canal, the infundibulum. 

 The cellular cavities of each lateral mass, thus walled in by the os planum on the 

 outer side and by the other bones already mentioned, are divided by a thin trans- 

 verse bony partition into two sets, which do not communicate with each other ; 

 they are termed the anterior and posterior ethmoidal cells or sinuses. The former, 

 more numerous, communicate with the frontal sinuses above and the middle 

 meatus below by means of a long, flexuous canal, the infundibulum ; the posterior, 

 less numerous, open into the superior meatus, and communicate (occasionally) 

 with the sphenoidal sinuses. 



Development. By three centres : one for the perpendicular lamella, and one 

 for each lateral mass. 



The lateral masses are first developed, ossific granules making their appearance 

 in the os planum between the fourth and fifth months of foetal life, and extending 

 into the spongy bones. At birth the bone consists of the two lateral masses, 

 which are small and ill-developed. During the first year after birth the perpen- 

 dicular and horizontal plates begin to ossify, from a single nucleus, and become 

 joined to the lateral masses about the beginning of the second year. The forma- 

 tion of the ethmoidal cells, which completes the bone, does not commence until 

 about the fourth or fifth year. 



Articulations. With fifteen bones : the sphenoid, two sphenoidal turbinated, 

 the frontal, and eleven of the face the two nasal, two superior maxillary, two 

 lachrymal, two palate, two inferior turbinated, and the vomer. No muscles are 

 attached to this bone. 



DEVELOPMENT OF THE CRANIUM. 



The early stages of the development of the cranium have already been described (see page 

 115). We have seen that it is formed from a layer of mesoblast, derived from the protovertebral 

 plates of the trunk, which is spread over the whole surface of the rudimentary brain. That 

 portion of this layer from which the bones of the skull are to be developed consists of a thin, 

 membranous capsule. 



Ossification commences in the roof, and is preceded by the deposition of a membranous 

 blastema upon the surface of the cerebral capsule, in which the ossifying process extends, the 

 primitive membranous capsule becoming the internal periosteum, and being ultimately blended 

 with the dura mater. Although the bones of the vertex of the skull appear before those at the 

 base, and make considerable progress in their growth, at birth ossification is more advanced in 

 the base, this portion of the skull forming a solid, immovable groundwork. 



