THE SKELETON 



255 



united (Figs. 120 and 125) and enclosing a large cavity, is filled 

 by the brain, the center of all mental activity. The cranium 

 is made of eight large bones, which in the adult are dove- 

 tailed together, the lines where they meet being called su- 

 tures. In childhood, 

 while the skull is grow- 

 ing, they are soft and 

 not so firmly joined, 

 though they touch each 

 other. If a young 

 baby's head be ex- 

 amined, there will be 

 found a soft spot in 

 the middle where the 

 bones have not yet 

 come together; Fig. 

 126. 



2. The facial bones, 

 thirteen in number, 

 form the face. They 

 enclose the eyes and 

 comprise the cheek 

 bones and upper jaw. 



3. The mandible, 

 forming the lower jaw, 



is a single bone hinged to the temporal bone of the cranium, 

 Fig. 125. The only movable bone in the skull, it is acted 

 on by powerful muscles attached to each side of the cranium 

 and is provided with a blade-like extension of its surface, so 

 that there may be a larger area for the attachment of the 

 muscle. 



It will be noticed that the skull is especially adapted to 

 the protection of several delicate organs of the nervous system. 

 The brain itself is entirely inclosed in bones which are so 

 thick that ordinary blows can not injure it. The eyes are 



FIG. 125. THE SKULL DISARTICULATED 



i.e. with the bones slightly pulled apart but 

 retaining their relative positions. 



