BONES OF THE SKULL. 
465 
prevents the temporal bone from being driven inwards, as it 
might have otherwise been, by a blow on the side of the head. 
618. In the base or floor of the cavity of the cranium are 
seen a number of apertures, which serve for the passage of 
the blood-vessels that supply the brain, and of the nerves that 
issue from it. One of these apertures, much larger than the 
rest, and situated in the occipital bone, gives passage to 
the Spinal Cord; and on each side of this aperture there is 
a large bony projection from the under surface, termed the 
condyle , by which the skull rests on the vertebral column, and 
is enabled to move forwards or backwards upon it. The head 
is nearly balanced upon this pivot; nevertheless, the portion 
situated in front of the joint is more heavy than that which 
is situated behind it, and is consequently not altogether 
counterpoised by the latter. Hence the muscles which, arising 
from the back and being attached to the occipital bone, tend 
to draw the head backwards, and thus to keep it upright, are 
more numerous and powerful than those which are situated in 
front of the vertebral column, and which tend to draw the head 
downwards and forwards; and when the former are relaxed, 
as in a person sleeping upright, the head has a tendency to 
fall forwards upon the chest. In no other animal is this joint 
situated so far forwards as in Man. As we descend the scale, 
we find it nearer and nearer to the back of the skull; and 
consequently the whole weight of the head bears, not directly 
upon the spine, but upon the muscles and ligaments by which 
it is attached to the vertebral column. 
619. On each side of the base of the cranium, we observe 
a large rounded projection, termed the mastoid. To this pro¬ 
jection (which we feel behind the lower part of the ear) is, 
attached on either side a powerful muscle, the sterno-mastoid 
( 23 , fig. 227), which passes downwards and towards the central 
line ; so that the two muscles nearly meet at the bottom of the 
neck, where they are attached to the upper edge of the breast¬ 
bone. These muscles, acting together, serve to draw the head 
forwards; but either of them acting separately will turn it to 
one side or the other. In front of these two projections of 
the skull, we notice the opening ta of the external ear; 
which, like the different chambers of the internal ear, is 
excavated in a portion of the temporal bone which is termed 
'petrous from its very dense and stony character. 
H H 
