464 
BONES OF THE SKULL. 
Motor Apparatus of Man:—Skeleton and Muscles. 
616. Before entering npon 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 
a f 
Fig. 220.— Human Skull. 
/, frontal bone; p, parietal; t, tem¬ 
poral ; o, occipital; s, sphenoid ; 
n, nasal; ms, superior maxillary; 
j, malar or cheek bone ; mi, in¬ 
ferior maxilla; na, anterior open¬ 
ing of the nose; ta, auditory aper¬ 
ture ; az, zygomatic arch ; a,b,c,d, 
lines indicating the facial angle. 
