156 THE SKELETON. 



in front of the superior meatus, is seen the convex surface of the middle turbi- 

 nated bone. It extends along the whole length of the inner surface of each 

 lateral mass ; its lower margin is free and thick, and its concavity, directed 

 outwards, 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 iufundibulum. 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 transverse 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, smaller but more numerous, 

 communicate with the frontal sinuses above, and the middle meatus below, by 

 means of a long flexuous cellular canal, the infundibulum ; the posterior, larger 

 but less numerous, open into the superior meatus, and communicate (occasion- 

 ally) 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 first 

 appearance in the os planum between the fourth and fifth months of fcetal 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 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 the two nasal, two superior maxillary, two 

 lachrymal, two palate, two inferior turbinated, and the vomer. 



DEVELOPMENT OF THE CRANIUM. 



The development of the cranium commences at a very early period, on account of the import- 

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

 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 continu- 

 ous 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 forepart, where it 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 retaining its membranous form. Ossification first takes place in the 

 roof, and is preceded bv the deposition of a membranous blastema upon the surface of the cere- 

 bral 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. 



THE FONTANELLES (Figs. 108, 109.) 



Before birth. Iho 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 con- 

 siderable size, and are termed t \wfnt<i >i<'ll> n, so called from the pulsations of the brain, which 

 arc perceptible at the anterior foritanelle. and were likened to the rising of water in a fountain. 

 The fontanelles are four in number, and correspond to the junction of the four angles of the 

 parietal with the contiguous bones. The anterior fontanelle is the largest, and corresponds to 

 the junction of the sagittal and coronal sutures; the posterior fontanelle. of smaller size, is 

 situated at the junction of the sagittal and lambdoid sutures; the two remaining ones are situated 

 at the inferior angles of the parietal bone. The latter are closed soon after birth ; the two at 

 the superior angles remain open longer: the posterior being closed in a few months after birth ; 

 the anterior remaining open until the first or second year. These spaces are gradually filled in 



