i8o 



HUMAN PHYSIOLOGY 



CHAP. 



one in which one bone turns or rotates upon another as 

 is shown in turning the forearm. In the hinge joint one 

 bone moves upon another in two directions only, back 

 and forth, like the blade of a jackknife or a door on its 

 hinges. The joints at the elbow, the knee, the fingers, 



and toes are examples of 

 this class. The saddle joint 

 is illustrated in the joint 

 between the thumb and the 

 wrist. In the gliding joint 

 one plane surface glides 

 on another, as the carpal 

 bones of the wrist and the 

 tarsals of the ankle. The 

 bones of the cranium are 

 so dovetailed together as 

 to permit of no real mo- 

 tion, and such joints are 

 known as sutures. Several 

 different kinds of sutures 

 occur among the bones of 

 the head. 



bones 



FIG. 96. The bones of the wrist. 

 The outer part cut away to show 

 the arrangement. (From Heitz- 

 mann by Zuppke.) 



1. Procure a sheep's knee and study the way the two bones fit 

 together and how they are held in place. 



2. Cut the several ligaments and trace each in both directions. 



3. Notice the kneepan and see how it serves to protect the joint. 



130. Posture. The bones of a young child are so 

 soft that they can be easily bent because the amount of 

 lime in them is small. That is why a child should not 

 be encouraged to stand or walk when too young. The 



