THE FONTANELLES. 



79 



divided by a thin transverse bony partition into two sets, which do not commu- 

 nicate with each other; they are termed the anterior and posterior vthmoidal cells; 

 the former, the smallest but the most numerous, communicate with the frontal 

 sinuses above, and the middle rneatus below, by means of a long flexuous cellular 

 canal, the infundibulum ; the posterior, the largest and least numerous, open into 

 the superior meatus, and communicate (occasionally) with the sphenoidnl 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 first 

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

 and afterwards in the spongy bones. At birth, the bone consists of the two 

 lateral masses, which are small and ill-developed; but when the perpendicular 

 and horizontal plates begin to ossify, as they do about the first year after birth, 

 the lateral masses become joined to the cribriform plate. The formation and 

 increase in the ethmoidal cells, which complete the formation of the bone, take 

 place about the fifth or sixth year. 



Articulations. "With fifteen bones; the sphenoid, two sphenoidal turbinated, the 

 frontal, and eleven of the face t\vo nasal, two superior maxillary, two lachrymal, 

 two palate, two inferior turbinated, and vomer. 



DEVELOPMENT OF THE CRANIUM. 



The development of the cranium takes place at a very early period, on account of the im- 

 portance of the organ it is intended to protect. In its most rudimentary state, it consists of a 

 thin membranous capsule; inclosing the cerebrum, and accurately moulded upon its surface. 

 This capsule is placed external to the dura mater, and in close contact with it; its walls are 

 continuous with the canal for the spinal cord, and the chorda dorsalis, or primitive part of the 

 vertebral column, is continued forwards, from the 



spine, along the base, to its fore part, where it v - 41 i 11 * TT *v v, ^ 



'. . , r, 1 ,, tig. 41. fekull at Birth, showing the Anterior 



terminates in a tapering point. The next step 



in the process of development is the formation 

 of cartilage. This is deposited in the base of 

 the skull, in two symmetrical segments, one on 

 either side of the median line ; these subsequently 

 coalesce, so as to inclose the chorda dorsalis : 

 the chief part of the cerebral capsule still re- 

 taining its membranous form. Ossification first 

 takes place in the roof, and is preceded by the 

 deposition of a membranous blastema upon the 

 surface of the cerebral capsule, in which the ossi- 

 fying 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 con- 

 siderable progress in their growth, at birth ossifi- 

 cation is more advanced in the base, this portion 

 of the skull forming a solid immovable ground- 

 work. 



and Posterior Fontanelles. 



THE FONTANELLES (figs. 41 and 42). 



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, at certain parts, are of consider- 

 able size, and are termed the fontanettes, so 

 called from the pulsations of the brain, which 

 resemble the rising of water at a fountain head. 

 The fontanelles are four in number, and corre- 

 spond to the junction of the four angles of the 

 parietal with the contiguous bones. The ante- 

 rior fontanelle is the largest, and corresponds to 

 the junction of the sagittal and coronal sutures ; 

 the posterior fontanelle, of smaller size, is situ- 

 ated at the junction of the sagittal and lambdoid 

 sutures ; the two remaining ones are situated at 



Fig. 42. The Lateral Fontanelles. 



