464 



BONES OF THE SKULL. 



Motor Apparatus of Man : Skeleton and Muscles. 



616. Before entering upon the examination of the various 

 movements of the lower animals, and of the means by which 

 these are effected, it will be useful to acquire a general know- 

 ledge of the structure of the Human Skeleton, and of the 

 uses of its several parts. The skeleton, which is formed by 

 the union of about 200 bones, is divided like the body into head., 

 trunk, and members. The bones of these parts will now be 

 separately described. 



617. The Head is composed of two parts, the cranium or 

 skull, and the face. The cranium (fig. 220) is a bony case of 

 oval form, occupying the upper and back part of the head, 



and serving for the protection of 

 the brain, which is lodged in its 

 cavity. Its walls are made-up of 

 eight bones : the frontal/ in the 

 region of the forehead; the two 

 parietal bones p, which occupy 

 the top and sides of the skull; 

 the two temporal bones t, which 

 form the walls of the temporal 

 region ; the occipital bone o at the 

 back of the head ; and the sphe- 

 noid s, and the ethmoid, which 

 assist in forming the floor of the 

 cavity. These bones are firmly 

 united to each other by sutures, 

 the character of which varies in 

 different parts of the cranium, 

 so that they are the better able to resist external violence. 

 Thus, a blow upon the top of the arch formed by the parietal 

 bones will tend to separate them from each other and from 

 the frontal bone, and to force asunder their lower borders. 

 Both these effects are resisted by the peculiarity of the suture 

 which unites different parts of the parietal bone to its neigh- 

 bours ; for at the top of the skull the bones are firmly held 

 together by the interlocking of the projections of each, whilst the 

 lower edge of the parietal bone is prevented from being driven 

 outwards by the overlapping edge of the temporal bones, which 

 form, as it were, a buttress to the arch. This same contrivance 



Fig. 220. HUMAN SKULL. 



/, frontal bone ; p, parietal ; t, tem- 

 poral ; o, occipital ; , sphenoid ; 

 n, nasal ; ms, superior maxillary ; 

 j, malar or cheek bone ; mi, in- 

 ferior maxilla; no, anterior open- 

 ing of the nose ; ta, auditory aper- 

 ture; az.zygpmatic arch ; a,b,c,d, 

 lines indicating the facial angle. 



