ENCEPHALON. 



75 



3 - 



The edges of the outer table are serrated, and so arranged as to be 

 accurately dovetailed into each other ; the tough fibrous texture of the 

 external plate being well adapted for such a junction. On the other 

 hand, the tabula vitrea, which, on account of its greater hardness, 

 would be liable to fracture and chip off, is merely united with its fellow 

 at the suture, by what is called harmony: the tables are merely placed 

 in contact. 



The precise object of the sutures is not 

 apparent. In the mode in which ossifi- 

 cation takes place in the bones of the 

 skull, the radii from different ossific 

 points must necessarily meet by the 

 "law 'of conjugation," in the progress of 

 ossification. This has, by many, been 

 esteemed the cause of the sutures ; but 

 the explanation is insufficient. Howso- 

 ever it may be, the kind of junction af- 

 fords a beautiful example of adaptation. 

 During the foetal state, the sutures do 

 not exist. They are fully formed in 

 youth ; are distinct in the adult age ; but 

 in after periods of life become entirely 

 obliterated, the bone then forming a solid 

 spheroid. It does not seem that after 

 the sutures are established, any displace- 

 ment of the bones can take place ; and 

 observation has shown, that they do not 

 possess much, if any, effect in putting a 

 limit to fractures. In all cases of severe 

 blows, the skull appears to resist as if it 

 were constituted of one piece. But the 

 separation of the skull into distinct bones, bone. 

 which have a membranous union, is of 

 striking advantage to the foetus in par- 

 turition. It enables the bones to overlap 

 each other ; and, in this way, to occupy 



a much smaller space than if ossification had united them as in after 

 life. It has, indeed, been imagined by some, that there is this advan- 

 tage in the pressure made on the brain by the investing bones, that 

 the foetus does not suffer from the violent efforts made to extrude the 

 child ; but, during the passage through the pelvis, is in a state of fortu- 

 nate insensibility; and pressure suddenly exerted upon the brain is 

 certainly attended with these effects, a fact, which has to be borne in 

 mind in the management of apoplexy, fracture of the skull, &c. 



The uses of the diploe, which separates the two tables of the skull, 

 are not equivocal. Composed of a cancellated structure, it is well 

 adapted to deaden the force of blows ; and as it forms, at the same 

 time, a bond of union and of separation, a fracture might be inflicted 

 upon the outer table of the skull, and yet be prevented from extending 

 to the tabula vitrea. Such cases have occurred, but they are rare. It 



Front view of the Skull. 



1. Frontal portion of the frontal bone. 

 2. Nasal tuberosity. 3. Supra-orbital 

 ridge. 4. Optic foramen. 5. Sphenoidal 

 fissure. 6. Spheno-maxillary fissure. 7. 

 Lachrymal fossa, and commencement of 

 the nasal duct. 8. Opening of the anterior 

 nares, and the vomer. 9. Infra-orbital 

 foramen. 10. Malar bone. 11. Symphysis 

 of the lower jaw. 12. Mental foramen. 

 13. Ramus of the lower jaw. 14. Parietal 

 15. Coronal suture. 16. Temporal 

 bone. 17. Squamous suture. 18. Great 

 alaof the sphenoid bone. 19. Commence- 

 ment of the temporal ridge. 20. Zygoma 

 of the temporal bone. 21. Mastoid pro- 



