THE SKELETON 35 



Structure of Joints. — The meeting of two bones forms 

 a joint (Exp. 4). Some of the joints are immovable. 

 The skull bones join in zigzag lines called sutures, formed 

 by the interlocking of sawlike projections (Fig. 35). These 

 immovable joints are necessary for the protection of the 

 brain, which is the most delicate of the organs. The brain 



attains almost its full size by the seventh , 



year of life; its bony case needs to grow ; yu^v^ 

 very little after that. The joints of the : i§^ .-' 

 pelvis are also immovable. All movable : *^v, .■ 

 joints have two cartilages, and as the bones "^r^-v 



turn, one cartilage slips over the other. «a 



There is an intermediate class of joints • ^SpS 

 found between the vertebrae and where the t /^D. 

 ribs join the breastbone. These joints de- FlG . 35 ._ sutures 

 pend for their motion upon the flexibility OF Skull - 

 and compressibility of their cartilages. They are called 

 mixed, or elastic, joints, and allow slight motion. Such a 

 joint has only one cartilage. 



Kinds of Movable Joints. — The movable joints are found 

 chiefly in the limbs. When one end of the bone is rounded 

 and fits into a cuplike hollow, the joint allows motion in 

 all directions, and is known as a ball-and-socket joint. The 

 hip joints and shoulder joints are examples. A hinge joint 

 allows motion in only two (opposite) directions ; for exam- 

 ple, the to-and-fro motion of the elbow. A pivot joint 

 allows a rotary motion ; examples, the first vertebra on 

 the second, one bone of forearm upon the other. A glid- 

 ing joint consists of several bones that slide upon one 

 another, as at the wrists and ankles. 



The Four Features presented by a Movable Joint (Fig. 

 36). — If not held in place, the bones would slip out of 

 their sockets, hence there are ligaments, or tough bands, 



