THE SUTURES. 59 



This structure appears to depend upon the pressure of the 

 temporal muscle externally, and the resistance of the brain 

 within, which makes the bones so thin, that their edges opposed 

 to each other are not sufiiciently thick to stop the extension of 

 their fibres in length, and thus to cause the common serrated 

 appearances of sutures ; but the narrow edge of the one bone 

 slides over the other. The squamous form is also more con- 

 venient here ; because such thin edges of bones, when accurately 

 applied one to another, have scarce any rough surface to 

 obstruct or hurt the muscle in its contraction ; which is still 

 i^irther provided for, by the manner of laying these edges on 

 each other ; for, in viewing their outside, we see the temporal 

 bones covering the sphenoidal and parietal, and this last sup- 

 porting the sphenoidal, while both mount on the frontal ; from 

 which disposition it is evident, that while the temporal muscle 

 is contracting, which is the only time it presses strongly in its 

 motion on the bones, its fibres slide easily over the external 

 edges. Another advantage of this structure is, that the whole 

 part is made stronger by the bones thus supporting each other. 



The indentation of the sutures are not so strongly marked 

 on the inside as on the outside of the cranium ; and sometimes 

 the bones seem to be joined by a straight line : in some 

 skulls, the internal surface is found entire, while the sutures 

 are manifest without. By this mechanism, there is no risk of 

 the sharp points of the bones growing inwards, since the 

 external serrae of each of the conjoined bones rest upon the 

 internal smooth-edged table of the other. 



The advantages of the sutures are these: 1. The cranium is 

 more easily formed and extended into a spherical figure, than 

 if it had been one continued bone. 2. The bones which are at 

 some distance from each other at birth, may then yield, and 

 allow to the head a change of shape, accommodated to the 

 passage it is engaged in. Whence, in difficult parturition, 

 the bones of the cranium, instead of being only brought 

 into contact, are sometimes made to mount one upon the 

 other. 



[The sutures which unite the bones of the cranium, are 



