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141 



The next change, observed in the structure of the cranium, is 

 the deposition of bone. The period, at which this takes place, 

 is rather indefinite. It always, however, commences some time 

 in the course of the third month, and earlier in the base than in 

 the vault. Bone will generally be found in the base, at the be- 

 ginning of the third month ; and, in the vault, towards the ter- 

 mination of the same month. 



The deposition of bone, in the base, is made in the cartilage, 

 which, as I have mentioned, previously existed there; but, in the vault, 

 in the medium, which connects together the two membranes of 

 the cranium : a medium, which I have already asserted, and which 

 I shall hereafter prove, to have no resemblance to cartilage. 

 There is, therefore, this remarkable difference between the struc- 

 ture of the base and vault, during their development, that, in 

 the former, cartilage precedes the formation of bone, but not in 

 the latter. These facts, with which Anatomists do not appear to 

 be generally acquainted,* will, on a future occasion, obtain con- 

 siderable attention. At present, I shall not take into consideration, 

 either the causes of cartilage preceding bone in the base, or of its 

 nonexistence in the vault, further than to observe, that both the 

 existence of cartilage in the base, at a period, when the rest of the 

 cranium is only membranous, and the commencement of the de- 

 position of bone in the same situation, before it is observed in the 

 vault, are arrangements, which appear to be necessary, for the 

 purpose of affording, at as early a period as possible, defence to 

 the base of the brain ; which, it is well known, cannot bear me- 

 chanical injury with so much impunity as the vault. 



The number of distinct points of ossification found in different 



* See Nesbit's Osteogeny. 



