80 



THE SKELETON. 



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 plate and crista galli begin to ossify, from a single nucleus, and become 

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

 form plate is ossified partly from the perpendicular plate and partly from the 

 lateral masses. The formation 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 lachry- 

 mal, 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. AVe 

 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 por- 

 tion 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. The ossification of the bones of the base takes place for the most part 

 in cartilage, and 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. 



The Fontanelles. 



Before birth the bones at the vertex and side of the skull are separated from each other by 

 membranous intervals in which bone is deficient. These intervals are principally found at the 

 four angles of the parietal bones. Hence there are six fontanelles. Their formation is due to 



FIG. 44. Skull at birth, showing the anterior 

 and posterior fontanelles. 



FIG. 45. The lateral fontanelles. 



the wave of ossification being circular and the bones quadrilateral ; the ossific matter first meets 

 at the margins of the bones, at the points nearest to their centres of ossification, and vacuities 



