THE SKELETON. 
63 
In the skull we find the bones united by a dovetailed arrange- 
ment (sutures), the edges of the bones are serrated, the projections 
of one bone fitting into depressions of the other, which are held 
together by the periosteum (the membrane covering bone). In 
joints such as these no motion is required. 
The connection between bones in the spinal column is an 
example of another form of joint. Here only a very limited amount 
of motion is required, and we find that the contiguous surfaces of 
the bones are connected by interposed cartilaginous tissue running 
from one bone to the other. 
Fig. 30. — Section of skull, after Owen. 
And lastly, we have movable joints. In these joints we have 
the surfaces of the extremities covered with a thin layer of cartilage, 
or grizzle (a firm elastic substance of a bluish colour : it helps to 
lessen friction, gives freedom to movement, and to diminish shock), 
which is surrounded by a capsular ligament, and this is attached to 
the edges of the articular surfaces. This ligament is lined by a 
membrane which secretes synovia, a lubricating fluid commonly 
called joint oil. The amount of motion permitted varies least in the 
knees, and greatest in the ball and socket joints, as shoulder, hip. 
Skull {see Figs. 29 and 30). — The construction of the skull 
differs widely from that of most animals. The cranial much exceeds 
the facial portion in size, its upper part forming an expanded dome, 
the remainder being formed of bony cell structure (air sinuses), 
