68 THE HUMAN BODY. 



occipital in the posterior part of the skull or occiput; the two 

 parietal bones, which form the side walls of the skull, and 

 contribute, with the frontal and occipital, to form its arch; 

 the two temporal bones occupy, as their name shows, the 

 region of the temples; the ethmoid, which owes its name to 

 the sieve-like plate of its upper surface ; and the sphenoid, so 

 called because it is wedged in between all the other bones 

 with which it articulates, and which rest upon it as upon the 

 keystone of an inverted arch, thus forming the base of the 

 skull on which the brain rests. The frontal, occipital, pari- 

 etal, and temporal bones are flat, formed of two plates of 

 ivory tissue internal and external tables between which is 

 a more or less thick layer of spongy tissue. 



The bones of the cranium are united by means of sutures 

 formed by the junction of the teeth of their serrated borders, 

 almost precisely like what is termed in architecture dove- 

 tailing. At birth the bones which form the arch of the skull 

 are united only by a membranous tissue, and their borders 

 overlap each other even on slight pressure in such a way as 

 to lessen the diameter of the head ; but although the sutures 

 are not yet developed, a part of the tooth-like processes 

 already exists in a rudimentary state. The membranous 

 intervals are greater at the point of union of the occipital 

 and frontal with the parietal bones; these spaces are called 

 the fontanelles. They are soon filled with bony tissue, and 

 at four years of age not a trace of them remains. About 

 the end of the third year the borders of the bones are cut 

 into fine notches or teeth, which increase in number till the 

 period of adolescence. Before this the suture which unites 

 the two halves of the frontal bone begins to disappear; and 

 later still, when the brain is fully developed, the other 

 bones gradually close together. 



The internal surface of the skull presents a series of 

 depressions, portions of the arch which have been called 

 fossa, and according to the bones which constitute them, the 

 frontal, occipital, and temporal fossce, and which correspond 

 to the projections which we see on the external surface. 

 There are also a great number of projections and depressions, 

 which to a certain extent are modelled to the surface of the 



