456 SUTURES :—MOVABLE ARTICULATIONS. 
in others it is replaced by a bevelling of the edges that are in 
contact, or by the reception of a ridge of one bone into a 
groove in the other. So firmly are the bones united in this 
manner, that it is difficult to separate them without breaking 
away some of their projecting parts; and in the skulls of 
old persons, the sutures are almost obliterated by the complete 
union between the adjacent bones. In the infant, on the 
other hand, the bones of the skull are only united to each 
other by a membranous substance ;. and there is a point at 
the top of the head, which is not even covered by a bony 
layer for some time after birth. It is only as the age ad- 
vances, and ossification becomes more complete (§$ 52), that 
firm bony union is effected. 
602. In several other articulations, the bones do not come 
into direct contact with each other, but are connected by an 
intervening layer of cartilage, and also by ligaments and 
other fibrous membranes encircling the articulations. The 
adjacent surfaces of the bones are flat, and have a slight 
gliding movement over one another; but the extent of 
motion permitted is very small. This kind of articulation 
exists between the bodies of the vertebrae of Man and the 
higher Vertebrata, between the bones of the pelvis, and some 
other parts. 
603. The proper movable articulations, by which the limbs 
are connected with the trunk, and their different divisions to 
each other, are those to which we commonly give the name of 
Joints. In these, the surfaces of the adjacent bones are not 
united in any other way than by the ligaments and muscles 
which surround them ; and they have a free gliding move- 
ment over each other. They are covered, it is true, by car- 
tilage ; this, however, does not pass from one bone to the 
other, as in the previous case, but forms a thin layer over the 
end of each, and presents a very smooth surface, which is 
secreted from the synovial membrane that envelops the joint 
lubricated by the fluid ($ 44). The beautiful smoothness of the 
surfaces of the joints, and the manner in which the bones are 
held together by the muscles and ligaments, is well seen by 
examining the knuckle-joint at the lower end of a leg of 
mutton (before being cooked), and the joint which connects 
it: with the bones of the haunch. These two joints are 
examples of the two principal varieties of freely-movable 
