266 Anatomy of Skeleton 



in number with the presumed dermal bones concerned : thus each half of the lower jaw, for example, 

 has only one centre instead of a number in agreement with the number of elements (angular, 

 splenial, etc.) that are supposed to have sunk into the deeper tissues of the mandibular arch 

 to make the bone. It seems doubtful, moreover, whether certain cartilage bones have not 

 been replaced by membrane bones which from their positions could not have come from the 

 surface. 



The chondro-cranium is replaced by the osseous cranium. The times of appearance 

 of the oss'fic centres vary in d'fferent bones ; but two main periods of activity in this 

 direction are to be noticed, the first about the end of the second month, or beginning 

 of the third month and the second about the fifth month. All the bones formed in 

 the cartilaginous base, excepting the special sense capsules, begin to ossify in the first 

 period, as do also all the membrane bones of the vault ; the lingula begins a little later, 

 in the fourth month. The bones formed in the special sense capsules, On the other 

 hand, belong to the second period, including the ethmoid, inferior turbinal, sphenoidal 

 turbinal, and petrous centres : the centre for the vertical plate of the ethmoid is even 

 later, occurring after birth. The lachrymal, nasal and vomer, membrane bones lying 

 against the nasal capsule, belong to the first period, as also does the tympanic bone, a 

 membrane bone applied to the periotic capsule in part. In the visceral skeleton there 

 is greater variation, the mandible showing centres in the fifth week, before the centres 

 of the maxillary process, the derivatives of which belong to the first period ; this 

 includes the maxilla, palate, internal pterygoid plate, and malar.* But the visceral 

 bones further back are much later than the second period, the tympano-hyal not 

 showing a centre until a month or so before birth and the stylo-hyal not until 

 some months after that event : the upper ends of these bars, however, come into the 

 second period by commencing to ossify as the small bones of the middle ear in the 

 fifth month. 



Summing up this account of the ossification, it may be said that all the membrane bones begin 

 to ossify in the first period, the cartilaginous cranial bones partly in the first and partly in the 

 second (special sense capsules), and the cartilaginous visceral bones considerably later. 



So it comes about that, save in the perpendicular plate of ethmoid and the styloid 

 process, all the bones of the skull are in process of formation for some time before birth, 

 although the various elements concerned have not joined with each other in all cases - 

 to make the bones as we see them in the older skull. Thus the bony post-, ex-, and 

 basi- occiput are distinct ; the basiocciput is separated by cartilage from the basi- 

 sphenoid, this partially from the presphenoid, the squamous from the petrous temporal, 

 and the two halves of the lower jaw are not joined in the middle line. In addition 

 the membrane bones of the cranial vault, though they grow rapidly, do not meet each 

 other over the quickly-growing brain, and at birth are separated by lines and areas of 

 fibrous tissue where the periosteal covering of the bones becomes directly continuous 

 with the dura mater lying immediately underneath, j The largest of these inter- 

 osseous fibrous areas occur at the angles of the parietal bone : the areas are termed 

 fontanelles, and these larger ones are the anterior and posterior median at the bregma 



* There is some reason to suppose that the lachrymal and vomer may be formed in tissues belonging 

 to the maxillary process where this is applied to the capsule, when their ossification periods would come into 

 line with the other " maxillary " bones. The outer pterygoid plate is ossified by extension from the sphenoid; 

 it is looked on as a muscular process, and thus as not having a morphological value equal to or as great as 

 that of the inner plate. 



t Before the sutures close the bones of the vault are thickest at their centres and thin away into the 

 fibrous tissue at their edges. After the edges come into contact they begin to increase in thickness and 

 ultimately become the thickest parts. At the same time the central parts of the bones become less pro- 

 minent. 



