456 SUTUEES : 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 



